[IR] Remove handling for removed ConstantExprs (NFC)

Author: nikic

llvm/lib/IR/Constants.cpp

@@ -3434,14 +3434,6 @@ Instruction *ConstantExpr::getAsInstruction() const {
 
   switch (getOpcode()) {
   case Instruction::Trunc:
-  case Instruction::ZExt:
-  case Instruction::SExt:
-  case Instruction::FPTrunc:
-  case Instruction::FPExt:
-  case Instruction::UIToFP:
-  case Instruction::SIToFP:
-  case Instruction::FPToUI:
-  case Instruction::FPToSI:
   case Instruction::PtrToInt:
   case Instruction::IntToPtr:
   case Instruction::BitCast: