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

[Jay Foad via llvm-commits]

jayfoad 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!

I cherry-picked your test, thanks. But I am no longer sure this is the best approach for fixing the problem. How about #79831 as an alternative?

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