[llvm] [AMDGPU] Fix an issue that wrong index is used in calculation of byte provider when the op is extract_vector_elt (PR #91697)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> > > Should include the Fixes: SWDEV-... comment in description
> > 
> > 
> > Do we want to include that information in upstream PR?
> 
> Yes

Specifically, the format "Fixes: SWDEV-xxxxxx" 

https://github.com/llvm/llvm-project/pull/91697