[ConstantFold] Avoid creation of undesirable binop

When commuting the operands, don't create a constant expression
for undesirable binops. Only invoke the constant folding function
in that case.

Author: nikic

llvm/lib/IR/ConstantFold.cpp

@@ -1084,7 +1084,9 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode, Constant *C1,
   } else if (isa<ConstantInt>(C1)) {
     // If C1 is a ConstantInt and C2 is not, swap the operands.
     if (Instruction::isCommutative(Opcode))
-      return ConstantExpr::get(Opcode, C2, C1);
+      return ConstantExpr::isDesirableBinOp(Opcode)
+                 ? ConstantExpr::get(Opcode, C2, C1)
+                 : ConstantFoldBinaryInstruction(Opcode, C2, C1);
   }
 
   if (ConstantInt *CI1 = dyn_cast<ConstantInt>(C1)) {

llvm/test/Transforms/InstCombine/pr32686.ll

@@ -8,8 +8,9 @@ define void @tinkywinky() {
 ; CHECK-LABEL: @tinkywinky(
 ; CHECK-NEXT:    [[PATATINO:%.*]] = load i8, ptr @a, align 1
 ; CHECK-NEXT:    [[TOBOOL_NOT:%.*]] = icmp eq i8 [[PATATINO]], 0
-; CHECK-NEXT:    [[TMP1:%.*]] = zext i1 [[TOBOOL_NOT]] to i32
-; CHECK-NEXT:    [[OR1:%.*]] = or i32 [[TMP1]], or (i32 zext (i1 icmp ne (ptr @a, ptr @b) to i32), i32 2)
+; CHECK-NEXT:    [[TMP1:%.*]] = or i1 [[TOBOOL_NOT]], icmp ne (ptr @a, ptr @b)
+; CHECK-NEXT:    [[TMP2:%.*]] = zext i1 [[TMP1]] to i32
+; CHECK-NEXT:    [[OR1:%.*]] = or i32 [[TMP2]], 2
 ; CHECK-NEXT:    store i32 [[OR1]], ptr @b, align 4
 ; CHECK-NEXT:    ret void
 ;