[llvm] AMDGPU: Handle demanded subvectors for readfirstlane (PR #128648)

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

This turned some of our bots red, e.g., https://lab.llvm.org/buildbot/#/builders/140/builds/18458

https://github.com/llvm/llvm-project/pull/128648