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

[Jannik Silvanus via cfe-commits]

jasilvanus wrote:

> There seem to be some clang test failures though that I'm currently looking into.

Solved: The clang failures were caused by me having `dxv` in `$PATH`, which implicitly enabled some extra `dxv`-based tests which also fail without this patch for me. 

Clang was invoking `dxv - -o -`, and `dxv` complained about `-` not being a file. Not sure whether automatically enabling extra tests based on `$PATH` is desirable behavior, but that's a different topic.

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