[llvm] [AMDGPU] Avoid invalid bitcasts for odd-width vector types (PR #196609)

[Steffen Larsen via llvm-commits]

steffenlarsen wrote:

@PlutoDog95 - Sorry for trying to steal your thunder. I only just realized that you had this fix open, after opening https://github.com/llvm/llvm-project/pull/201520. That said, our solutions are slightly different, so I'm open to sticking to whichever reviewers think is the more robust.

https://github.com/llvm/llvm-project/pull/196609