[llvm] [InstCombine] Remove redundant alignment assumptions. (PR #123348)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Can this ever delete unrecoverable information?

The main case where we can lose info is if we derive the alignment from an argument and later inline the function; then the align arg attribute will be gone. Updated to only do this for pointers not derived from an argument.

Without support for GEP recurrences, we would also lose information in some cases when using the alignment of a call result, but that should be fixed with https://github.com/llvm/llvm-project/pull/123518

https://github.com/dtcxzyw/llvm-opt-benchmark/pull/1984/files shows the impact with with both #123518 and this PR

https://github.com/llvm/llvm-project/pull/123348