Increase accuracy of atan2pi for half

Change-Id: Ia8704610e0ba5067fcac35c82fdc97ad8de67976

Author: b-sumner

amd/device-libs/ocml/src/atanpiredH.cl

@@ -10,9 +10,13 @@
 CONSTATTR half
 MATH_PRIVATE(atanpired)(half v)
 {
+    const half ch = 0x1.45cp-2h;
+    const half cl = 0x1.85cp-13h;
     half t = v * v;
-    half z = MATH_MAD(t, MATH_MAD(t, MATH_MAD(t,
-                 -0x1.ef4p-7h, 0x1.a44p-5h), -0x1.ac8p-4h), 0x1.46p-2h);
-    return v * z;
+    half y = MATH_MAD(t, MATH_MAD(t, MATH_MAD(t, 0x1.f04p-8h, -0x1.dfp-6h), 0x1.e3p-5h), -0x1.b08p-4h);
+    half ph = v * ch;
+    half pl = MATH_MAD(v, ch, -ph);
+    half r = MATH_MAD(v, MATH_MAD(t, y, cl), pl) + ph;
+    return r;
 }