[llvm] [NFC][PowerPC] Add test to check lanemasks for subregisters. (PR #94363)

[Nico Weber via llvm-commits]

nico wrote:

This breaks tests: http://45.33.8.238/linux/140640/step_11.txt

Please take a look and revert for now if it takes a while to fix.

https://github.com/llvm/llvm-project/pull/94363