Revert "[IR] Mark mul and ashr const exprs as undesirable"

This reverts commit 0a362f1.

This causes test failures in flang.

Author: nikic

llvm/lib/IR/Constants.cpp

@@ -2133,13 +2133,13 @@ bool ConstantExpr::isDesirableBinOp(unsigned Opcode) {
   case Instruction::FRem:
   case Instruction::And:
   case Instruction::Or:
-  case Instruction::Mul:
-  case Instruction::AShr:
     return false;
   case Instruction::Add:
   case Instruction::Sub:
+  case Instruction::Mul:
   case Instruction::Shl:
   case Instruction::LShr:
+  case Instruction::AShr:
   case Instruction::Xor:
     return true;
   default: