[llvm] [InstCombine] Try optimizing with knownbits which determined from Cond (PR #91762)

[via llvm-commits]

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@@ -590,13 +783,13 @@ define i32 @n5(i32 %arg) {
 ; CHECK-LABEL: @n5(
 ; CHECK-NEXT:    [[T:%.*]] = and i32 [[ARG:%.*]], 2
 ; CHECK-NEXT:    [[T1:%.*]] = icmp eq i32 [[T]], 0
-; CHECK-NEXT:    [[T2:%.*]] = and i32 [[ARG]], 2
+; CHECK-NEXT:    [[T2:%.*]] = and i32 [[ARG]], 3
 ; CHECK-NEXT:    [[T3:%.*]] = select i1 [[T1]], i32 [[T2]], i32 1
 ; CHECK-NEXT:    ret i32 [[T3]]
 ;
   %t = and i32 %arg, 2
   %t1 = icmp eq i32 %t, 0
-  %t2 = and i32 %arg, 2 ; 2 instead of 1
+  %t2 = and i32 %arg, 3 ; 3 instead of 2
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ParkHanbum wrote:

"%t2 = and i32 %arg, 2" has same knownbits with %t. and because it be in trueval, %t will become 0. therefore %t2 also become 0. 

these n series tests are written to keep false cases for AND bit test. so, it was miss written IMO.

https://github.com/llvm/llvm-project/pull/91762