[InstCombine] Remove AllOnes fallbacks in getMaskedTypeForICmpPair()

getMaskedTypeForICmpPair() tries to model non-and operands as
x & -1. However, this can end up confusing the matching logic,
by picking the -1 operand as the "common" operand, resulting in
a successful, but useless, match. This is what causes commutation
failures for some of the optimizations driven by this function.

Fix this by removing this -1 fallback entirely. We don't seem to
have any test coverage that demonstrates why it would be needed.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp

@@ -270,6 +270,10 @@ static std::optional<std::pair<unsigned, unsigned>> getMaskedTypeForICmpPair(
       E = R2;
       Ok = true;
     }
+
+    // Avoid matching against the -1 value we created for unmasked operand.
+    if (Ok && match(A, m_AllOnes()))
+      Ok = false;
   }
 
   // Bail if RHS was a icmp that can't be decomposed into an equality.

llvm/test/Transforms/InstCombine/bit-checks.ll

@@ -809,12 +809,10 @@ define i32 @main7a_logical(i32 %argc, i32 %argc2, i32 %argc3) {
 define i32 @main7b(i32 %argc, i32 %argc2, i32 %argc3x) {
 ; CHECK-LABEL: @main7b(
 ; CHECK-NEXT:    [[ARGC3:%.*]] = mul i32 [[ARGC3X:%.*]], 42
-; CHECK-NEXT:    [[AND1:%.*]] = and i32 [[ARGC:%.*]], [[ARGC2:%.*]]
-; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i32 [[ARGC2]], [[AND1]]
-; CHECK-NEXT:    [[AND2:%.*]] = and i32 [[ARGC]], [[ARGC3]]
-; CHECK-NEXT:    [[TOBOOL3:%.*]] = icmp ne i32 [[ARGC3]], [[AND2]]
-; CHECK-NEXT:    [[AND_COND_NOT:%.*]] = or i1 [[TOBOOL]], [[TOBOOL3]]
-; CHECK-NEXT:    [[STOREMERGE:%.*]] = zext i1 [[AND_COND_NOT]] to i32
+; CHECK-NEXT:    [[TMP1:%.*]] = or i32 [[ARGC2:%.*]], [[ARGC3]]
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[ARGC:%.*]], [[TMP1]]
+; CHECK-NEXT:    [[AND_COND:%.*]] = icmp ne i32 [[TMP2]], [[TMP1]]
+; CHECK-NEXT:    [[STOREMERGE:%.*]] = zext i1 [[AND_COND]] to i32
 ; CHECK-NEXT:    ret i32 [[STOREMERGE]]
 ;
   %argc3 = mul i32 %argc3x, 42 ; thwart complexity-based canonicalization
@@ -850,12 +848,10 @@ define i32 @main7b_logical(i32 %argc, i32 %argc2, i32 %argc3) {
 define i32 @main7c(i32 %argc, i32 %argc2, i32 %argc3x) {
 ; CHECK-LABEL: @main7c(
 ; CHECK-NEXT:    [[ARGC3:%.*]] = mul i32 [[ARGC3X:%.*]], 42
-; CHECK-NEXT:    [[AND1:%.*]] = and i32 [[ARGC2:%.*]], [[ARGC:%.*]]
-; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i32 [[ARGC2]], [[AND1]]
-; CHECK-NEXT:    [[AND2:%.*]] = and i32 [[ARGC3]], [[ARGC]]
-; CHECK-NEXT:    [[TOBOOL3:%.*]] = icmp ne i32 [[ARGC3]], [[AND2]]
-; CHECK-NEXT:    [[AND_COND_NOT:%.*]] = or i1 [[TOBOOL]], [[TOBOOL3]]
-; CHECK-NEXT:    [[STOREMERGE:%.*]] = zext i1 [[AND_COND_NOT]] to i32
+; CHECK-NEXT:    [[TMP1:%.*]] = or i32 [[ARGC2:%.*]], [[ARGC3]]
+; CHECK-NEXT:    [[TMP2:%.*]] = and i32 [[ARGC:%.*]], [[TMP1]]
+; CHECK-NEXT:    [[AND_COND:%.*]] = icmp ne i32 [[TMP2]], [[TMP1]]
+; CHECK-NEXT:    [[STOREMERGE:%.*]] = zext i1 [[AND_COND]] to i32
 ; CHECK-NEXT:    ret i32 [[STOREMERGE]]
 ;
   %argc3 = mul i32 %argc3x, 42 ; thwart complexity-based canonicalization