[ConstantFolding] Fix wrong folding of intrinsic 'convert.from.fp16'.

Function 'ConstantFoldScalarCall' (in ConstantFolding.cpp) works under the
wrong assumption that a call to 'convert.from.fp16' returns a value of
type 'float'.
However, intrinsic 'convert.from.fp16' can be overloaded; for example, we
can call 'convert.from.fp16.f64' to convert from half to double; etc.

Before this patch, the following example would have triggered an assertion
failure in opt (with -constprop):

```
define double @foo() {
entry:
  %0 = call double @llvm.convert.from.fp16.f64(i16 0)
  ret double %0
}
```

This patch fixes the problem in ConstantFolding.cpp. When folding a call to
convert.from.fp16, we perform a different kind of conversion based on the call
return type.

Added test 'Transform/ConstProp/convert-from-fp16.ll'.

Differential Revision: http://reviews.llvm.org/D9771


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237377 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/ConstantFolding.cpp

@@ -1543,8 +1543,8 @@ static Constant *ConstantFoldScalarCall(StringRef Name, unsigned IntrinsicID,
         APFloat Val(APFloat::IEEEhalf, Op->getValue());
 
         bool lost = false;
-        APFloat::opStatus status =
-          Val.convert(APFloat::IEEEsingle, APFloat::rmNearestTiesToEven, &lost);
+        APFloat::opStatus status = Val.convert(
+            Ty->getFltSemantics(), APFloat::rmNearestTiesToEven, &lost);
 
         // Conversion is always precise.
         (void)status;