[llvm] [AMDGPU][True16][CodeGen] use vgpr16 for zext patterns (PR #153894)

[Jan Patrick Lehr via llvm-commits]

jplehr wrote:

Hi,
this one appears to result in a failure on our buildbot: https://lab.llvm.org/buildbot/#/builders/123/builds/25421
I reverted this patch locally and the issue goes away. 
It seems to only be an issue when building for `gfx1100`. Can someone please take a look at this?
And can we maybe revert in the meantime? 
Adding @ronlieb and @Kewen12 for awareness.

https://github.com/llvm/llvm-project/pull/153894