[llvm] AMDGPU: Reduce readfirstlane for single demanded vector element (PR #128647)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> LLVM Buildbot has detected a new failure on builder `sanitizer-aarch64-linux-bootstrap-hwasan` running on `sanitizer-buildbot11` while building `llvm` at step 2 "annotate".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/55/builds/7930
> 
> Here is the relevant piece of the build log for the reference

Fixed  by5c375c3283fcd2bf4f98fe8627658e056e25dc44

https://github.com/llvm/llvm-project/pull/128647