assigned to @LebedevRI

Nice! Thank you for reporting!
Really happy that the tool, as it is being expanded,
uncovers yet more and more horrors of all this pattern-matching!

And same for llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sle-to-icmp-sle.ll
So basically all the folds in that group when the comparison predicate is signed.

For reference, the commit that introduced this specific test change was:
https://reviews.llvm.org/rL337105

We missed a precondition. Not sure if it's worth using value tracking (isKnownNonZero) to preserve the cases that could still be reduced:

https://rise4fun.com/Alive/5gkT

Pre: C1 != 0
%tmp0 = lshr i8 255, C1
%tmp1 = and i8 %tmp0, %x
%ret = icmp sgt i8 %x, %tmp1
=>
%tmp0 = lshr i8 255, C1
%ret = icmp sgt i8 %x, %tmp0

For reference, the commit that introduced this specific test change was:
https://reviews.llvm.org/rL337105
Yep.

We missed a precondition.
Not so much missed. I did check the original fold with constants,
and then somehow managed to be completely unbothered to check the fold with
the lshr, and didn't think of it even.

Not sure if it's worth using value tracking
(isKnownNonZero) to preserve the cases that could still be reduced:
Could be done as a new feature.

https://rise4fun.com/Alive/5gkT

Pre: C1 != 0
%tmp0 = lshr i8 255, C1
%tmp1 = and i8 %tmp0, %x
%ret = icmp sgt i8 %x, %tmp1
=>
%tmp0 = lshr i8 255, C1
%ret = icmp sgt i8 %x, %tmp0

Checked other folds there, with other edge-cases (0, -1), everything else seems fine now..
Thanks for the bugreport (: