[ConstantFold] Use getFltSemantics instead of manually checking the type

Simplifies the code and makes fpext/fptrunc constant folding not crash
when the result is bf16.

Author: d0k

llvm/lib/IR/ConstantFold.cpp

@@ -435,14 +435,8 @@ Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
     if (ConstantFP *FPC = dyn_cast<ConstantFP>(V)) {
       bool ignored;
       APFloat Val = FPC->getValueAPF();
-      Val.convert(DestTy->isHalfTy() ? APFloat::IEEEhalf() :
-                  DestTy->isFloatTy() ? APFloat::IEEEsingle() :
-                  DestTy->isDoubleTy() ? APFloat::IEEEdouble() :
-                  DestTy->isX86_FP80Ty() ? APFloat::x87DoubleExtended() :
-                  DestTy->isFP128Ty() ? APFloat::IEEEquad() :
-                  DestTy->isPPC_FP128Ty() ? APFloat::PPCDoubleDouble() :
-                  APFloat::Bogus(),
-                  APFloat::rmNearestTiesToEven, &ignored);
+      Val.convert(DestTy->getFltSemantics(), APFloat::rmNearestTiesToEven,
+                  &ignored);
       return ConstantFP::get(V->getContext(), Val);
     }
     return nullptr; // Can't fold.

llvm/test/Transforms/InstSimplify/ConstProp/cast.ll

@@ -62,3 +62,11 @@ define <3 x half> @nan_v3f64_trunc() {
   %f = fptrunc <3 x double> <double 0x7FF0020000000000, double 0x7FF003FFFFFFFFFF, double 0x7FF8000000000001> to <3 x half>
   ret <3 x half> %f
 }
+
+define bfloat @nan_bf16_trunc() {
+; CHECK-LABEL: @nan_bf16_trunc(
+; CHECK-NEXT:    ret bfloat 0xR7FC0
+;
+  %f = fptrunc double 0x7FF0000000000001 to bfloat
+  ret bfloat %f
+}