[llvm] [clang] [IR] Fix GEP offset computations for vector GEPs (PR #75448)

[Jannik Silvanus via cfe-commits]

jasilvanus wrote:

> Alternative would be to forbid GEP indexing into vectors entirely.

I agree that it would be better if there just were no vector GEPs, but that breaks importing older modules, and that cannot be easily auto-upgraded (convert to byte-GEPs?). Even worse, DXIL uses different vector semantics where alignment in vectors is respected, so importing DXIL already is a challenge, but if vector GEPs in DXIL were to be replaced by byte-GEPs as part of auto-upgrade, preserving DXIL semantics becomes difficult.

https://github.com/llvm/llvm-project/pull/75448