[IR] Mark FP casts constant exprs as undesirable

We will no longer generate these constant expressions by default,
but they can still be explicitly created.

Author: nikic

llvm/lib/IR/Constants.cpp

@@ -2270,14 +2270,14 @@ bool ConstantExpr::isDesirableCastOp(unsigned Opcode) {
   switch (Opcode) {
   case Instruction::ZExt:
   case Instruction::SExt:
-    return false;
-  case Instruction::Trunc:
   case Instruction::FPTrunc:
   case Instruction::FPExt:
   case Instruction::UIToFP:
   case Instruction::SIToFP:
   case Instruction::FPToUI:
   case Instruction::FPToSI:
+    return false;
+  case Instruction::Trunc:
   case Instruction::PtrToInt:
   case Instruction::IntToPtr:
   case Instruction::BitCast: