[DirectX] make firstbitlow intrinsic use first argument instead of return for overload type (llvm#145350)

fixes llvm#144966
Easy fix just add `dx_firstbitlow` to
`DirectXTTIImpl::isTargetIntrinsicWithOverloadTypeAtArg`

Author: farzonl

llvm/lib/Target/DirectX/DirectXTargetTransformInfo.cpp

@@ -30,6 +30,7 @@ bool DirectXTTIImpl::isTargetIntrinsicWithOverloadTypeAtArg(Intrinsic::ID ID,
   switch (ID) {
   case Intrinsic::dx_asdouble:
   case Intrinsic::dx_isinf:
+  case Intrinsic::dx_firstbitlow:
     return OpdIdx == 0;
   default:
     return OpdIdx == -1;

llvm/test/CodeGen/DirectX/firstbitlow.ll

@@ -41,7 +41,20 @@ entry:
   ret <4 x i32> %2
 }
 
-declare i32 @llvm.dx.firstbitlow.i16(i16)
-declare i32 @llvm.dx.firstbitlow.i32(i32)
-declare i32 @llvm.dx.firstbitlow.i64(i64)
-declare <4 x i32> @llvm.dx.firstbitlow.v4i32(<4 x i32>)
+define noundef <4 x i32> @test_firstbitlow_vec4_i16(<4 x i16> noundef %a)  {
+entry:
+  ; CHECK: [[ee0:%.*]] = extractelement <4 x i16> %a, i64 0
+  ; CHECK: [[ie0:%.*]] = call i32 @dx.op.unaryBits.i16(i32 32, i16 [[ee0]])
+  ; CHECK: [[ee1:%.*]] = extractelement <4 x i16> %a, i64 1
+  ; CHECK: [[ie1:%.*]] = call i32 @dx.op.unaryBits.i16(i32 32, i16 [[ee1]])
+  ; CHECK: [[ee2:%.*]] = extractelement <4 x i16> %a, i64 2
+  ; CHECK: [[ie2:%.*]] = call i32 @dx.op.unaryBits.i16(i32 32, i16 [[ee2]])
+  ; CHECK: [[ee3:%.*]] = extractelement <4 x i16> %a, i64 3
+  ; CHECK: [[ie3:%.*]] = call i32 @dx.op.unaryBits.i16(i32 32, i16 [[ee3]])
+  ; CHECK: insertelement <4 x i32> poison, i32 [[ie0]], i64 0
+  ; CHECK: insertelement <4 x i32> %{{.*}}, i32 [[ie1]], i64 1
+  ; CHECK: insertelement <4 x i32> %{{.*}}, i32 [[ie2]], i64 2
+  ; CHECK: insertelement <4 x i32> %{{.*}}, i32 [[ie3]], i64 3
+  %2 = call <4 x i32> @llvm.dx.firstbitlow.v4i16(<4 x i16> %a)
+  ret <4 x i32> %2
+}