Use a type-appropriate 2.0 for reflect (microsoft#3998)

The reflection calculation involves a multiply by two that was always
float, which caused an internal assert when the variable was not float.
Now the constant uses the type of the other operand

Author: pow2clk

lib/HLSL/HLOperationLower.cpp

@@ -2147,7 +2147,7 @@ Value *TranslateReflect(CallInst *CI, IntrinsicOp IOP, OP::OpCode op,
   unsigned vecSize = VT->getNumElements();
   Value *dot = TranslateFDot(i, n, vecSize, hlslOP, Builder);
   // 2 * dot (i, n).
-  dot = Builder.CreateFMul(hlslOP->GetFloatConst(2), dot);
+  dot = Builder.CreateFMul(ConstantFP::get(dot->getType(), 2.0), dot);
   // 2 * n * dot(i, n).
   Value *vecDot = Builder.CreateVectorSplat(vecSize, dot);
   Value *nMulDot = Builder.CreateFMul(vecDot, n);

tools/clang/test/HLSLFileCheck/hlsl/intrinsics/compound/reflect_min16.hlsl

@@ -0,0 +1,13 @@
+// RUN: %dxc -T ps_6_0 %s | FileCheck %s
+
+// Ensure no validation errors
+// CHECK-NOT: error
+
+// Ensure the reflect generated intrinsics are present
+// CHECK: call half @dx.op.dot3.f16
+// CHECK: fmul fast half {{.*}}, 0xH4000
+
+
+min16float3 main(min16float3 i : I, min16float3 n: N) : SV_Target {
+  return reflect(i,n);
+}