Only overload vector operators for convertible types

This is necessary to avoid errors about ambiguous overloads in code such as
````

struct Point {
	float4 pos;
	float mass;
};

template<typename T>
Point operator+(Point const& p, T const& v)
{
	return Point{p.pos + v, p.mass};
}

int main() {
	float4 v = make_float4(0, 1, 2, 3);
	Point p{make_float4(3, 2, 1, 0), 1.0f};

	Point q = p + v;
}
````
when building with the host compiler.

Closes hipamd issue #4

Author: Oblomov

include/hip/amd_detail/amd_hip_vector_types.h

@@ -955,6 +955,13 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
         }
     };
 
+    // comfort type to only enable an operator if U can be converted to
+    // a HIP_vector_type<T, N>
+    template<typename U, typename T, unsigned int n,
+        typename R=HIP_vector_type<T, n> /* operator return value */>
+    using enable_if_convertible = typename
+        std::enable_if<std::is_convertible<U, HIP_vector_type<T, n>>::value, R>::type;
+
     template<typename T, unsigned int n>
     __HOST_DEVICE__
     inline
@@ -968,7 +975,7 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    HIP_vector_type<T, n> operator+(
+    enable_if_convertible<U, T, n> operator+(
         const HIP_vector_type<T, n>& x, U y) noexcept
     {
         return HIP_vector_type<T, n>{x} += HIP_vector_type<T, n>{y};
@@ -977,7 +984,7 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    HIP_vector_type<T, n> operator+(
+    enable_if_convertible<U, T, n> operator+(
         U x, const HIP_vector_type<T, n>& y) noexcept
     {
         return HIP_vector_type<T, n>{x} += y;
@@ -996,7 +1003,7 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    HIP_vector_type<T, n> operator-(
+    enable_if_convertible<U, T, n> operator-(
         const HIP_vector_type<T, n>& x, U y) noexcept
     {
         return HIP_vector_type<T, n>{x} -= HIP_vector_type<T, n>{y};
@@ -1005,7 +1012,7 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    HIP_vector_type<T, n> operator-(
+    enable_if_convertible<U, T, n> operator-(
         U x, const HIP_vector_type<T, n>& y) noexcept
     {
         return HIP_vector_type<T, n>{x} -= y;
@@ -1024,7 +1031,7 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    HIP_vector_type<T, n> operator*(
+    enable_if_convertible<U, T, n> operator*(
         const HIP_vector_type<T, n>& x, U y) noexcept
     {
         return HIP_vector_type<T, n>{x} *= HIP_vector_type<T, n>{y};
@@ -1033,7 +1040,7 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    HIP_vector_type<T, n> operator*(
+    enable_if_convertible<U, T, n> operator*(
         U x, const HIP_vector_type<T, n>& y) noexcept
     {
         return HIP_vector_type<T, n>{x} *= y;
@@ -1052,7 +1059,7 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    HIP_vector_type<T, n> operator/(
+    enable_if_convertible<U, T, n> operator/(
         const HIP_vector_type<T, n>& x, U y) noexcept
     {
         return HIP_vector_type<T, n>{x} /= HIP_vector_type<T, n>{y};
@@ -1061,7 +1068,7 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    HIP_vector_type<T, n> operator/(
+    enable_if_convertible<U, T, n> operator/(
         U x, const HIP_vector_type<T, n>& y) noexcept
     {
         return HIP_vector_type<T, n>{x} /= y;
@@ -1090,15 +1097,15 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    bool operator==(const HIP_vector_type<T, n>& x, U y) noexcept
+    enable_if_convertible<U, T, n, bool> operator==(const HIP_vector_type<T, n>& x, U y) noexcept
     {
         return x == HIP_vector_type<T, n>{y};
     }
     template<typename T, unsigned int n, typename U>
     __HOST_DEVICE__
     inline
     constexpr
-    bool operator==(U x, const HIP_vector_type<T, n>& y) noexcept
+    enable_if_convertible<U, T, n, bool> operator==(U x, const HIP_vector_type<T, n>& y) noexcept
     {
         return HIP_vector_type<T, n>{x} == y;
     }
@@ -1116,15 +1123,15 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
     __HOST_DEVICE__
     inline
     constexpr
-    bool operator!=(const HIP_vector_type<T, n>& x, U y) noexcept
+    enable_if_convertible<U, T, n, bool> operator!=(const HIP_vector_type<T, n>& x, U y) noexcept
     {
         return !(x == y);
     }
     template<typename T, unsigned int n, typename U>
     __HOST_DEVICE__
     inline
     constexpr
-    bool operator!=(U x, const HIP_vector_type<T, n>& y) noexcept
+    enable_if_convertible<U, T, n, bool> operator!=(U x, const HIP_vector_type<T, n>& y) noexcept
     {
         return !(x == y);
     }