[IR] Mark mul constant expression as undesirable

This is a (very belated) reland of 0a362f1,
which I originally reverted due to flang test failures.

This marks mul constant expressions as undesirable, which means that
we will no longer create them by default, but they can still be
created explicitly.

Part of:
https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179

Author: nikic

llvm/lib/IR/Constants.cpp

@@ -2395,10 +2395,10 @@ bool ConstantExpr::isDesirableBinOp(unsigned Opcode) {
   case Instruction::LShr:
   case Instruction::AShr:
   case Instruction::Shl:
+  case Instruction::Mul:
     return false;
   case Instruction::Add:
   case Instruction::Sub:
-  case Instruction::Mul:
   case Instruction::Xor:
     return true;
   default:

llvm/test/Transforms/InstCombine/add2.ll

@@ -325,7 +325,8 @@ define i16 @mul_add_to_mul_9(i16 %a) {
 
 define i32 @shl_add_to_shl_constexpr() {
 ; CHECK-LABEL: @shl_add_to_shl_constexpr(
-; CHECK-NEXT:    ret i32 mul (i32 ptrtoint (ptr @g to i32), i32 4)
+; CHECK-NEXT:    [[ADD:%.*]] = shl i32 ptrtoint (ptr @g to i32), 2
+; CHECK-NEXT:    ret i32 [[ADD]]
 ;
   %shl = shl i32 ptrtoint (ptr @g to i32), 1
   %add = add i32 %shl, %shl

llvm/test/Transforms/NewGVN/2009-11-12-MemDepMallocBitCast.ll

@@ -5,7 +5,7 @@
 
 define i64 @test() {
 ; CHECK-LABEL: define i64 @test() {
-; CHECK-NEXT:    [[TMP1:%.*]] = tail call ptr @malloc(i64 mul (i64 ptrtoint (ptr getelementptr (i64, ptr null, i64 1) to i64), i64 4))
+; CHECK-NEXT:    [[TMP1:%.*]] = tail call ptr @malloc(i64 mul (i64 4, i64 ptrtoint (ptr getelementptr (i64, ptr null, i64 1) to i64)))
 ; CHECK-NEXT:    store i8 42, ptr [[TMP1]], align 1
 ; CHECK-NEXT:    [[Y:%.*]] = load i64, ptr [[TMP1]], align 4
 ; CHECK-NEXT:    ret i64 [[Y]]