[ValueTracking] Check that alignment is non-zero in computeKnownBitsFromAssume (PR48713).

[spatel]

Adding more potential reviewers based on D88669 (where this code was added).

[spatel]

Please upload patch with context:
https://llvm.org/docs/Phabricator.html#requesting-a-review-via-the-web-interface

[rmansfield]

Updated diff with context

[jdoerfert]

No doubt there is a problem here somewhere, not sure if this is the best fix though or just hides it one level deeper.

I wonder if align(0) in the input is not already broken.
I'm trying to see if the Clang ppl are interested in changing the behavior that I think is at fault here: https://godbolt.org/z/jo4fx7
When we warn, we should not create the alignment assumption because in IR things depend on the fact that it is a power of two.

If we modify clang we should teach the verifier about the test case above and introduce an assertion isPowerOfTwo(RK.ArgValue).