Use SExt instead of ZExt for frexp. (#2395)

* Use SExt instead of ZExt for frexp.

lib/HLSL/HLOperationLower.cpp

@@ -1981,7 +1981,7 @@ Value *TranslateFrexp(CallInst *CI, IntrinsicOp IOP, OP::OpCode opcode,
 
   // bool ne = val != 0;
   Value *notZero = Builder.CreateFCmpUNE(val, zeroVal);
-  notZero = Builder.CreateZExt(notZero, dstTy);
+  notZero = Builder.CreateSExt(notZero, dstTy);
 
   Value *intVal = Builder.CreateBitCast(val, dstTy);
   // temp = intVal & exponentMask;

tools/clang/test/CodeGenHLSL/batch/expressions/intrinsics/frexp.hlsl

@@ -0,0 +1,18 @@
+// RUN: %dxc -E main -T ps_6_2 %s | FileCheck %s
+
+// Make sure frexp generate code pattern.
+// CHECK:bitcast float {{.*}} to i32
+// CHECK:and i32 {{.*}}, 2139095040
+// CHECK:add {{.*}}, -1056964608
+// CHECK:ashr {{.*}}, 23
+// CHECK:sitofp
+// CHECK:and i32 {{.*}}, 8388607
+// CHECK:or i32 {{.*}}, 1056964608
+// CHECK:fadd
+
+float main(float a:A) : SV_Target {
+  float b;
+  float c = frexp ( a , b );
+
+  return b+c;
+}