[llvm] [TableGen] Fix computeRegUnitLaneMasks for ad hoc aliasing (PR #79835)

[Kazushi Marukawa via llvm-commits]

kaz7 wrote:

Hi @jayfoad , thank you for this PR.  I prepared a test for this PR like you asked me in #79642.  What is the best way for you to merge the test I made?  1) merge this PR as usual, and I submit another PR for a test?  2) cherry-pick my test to this PR?  Either is fine.  Please let me know when you have time.  Thanks!

The test is available at https://github.com/kaz7/llvm-project/commit/b8d3ad4f9ece967ae0512b974893af95575d1af9.

https://github.com/llvm/llvm-project/pull/79835