[llvm] [TableGen] Fix computeRegUnitLaneMasks for ad hoc aliasing (PR #79462)

[Kazushi Marukawa via llvm-commits]

kaz7 wrote:

Thank you!

I've noticed this PR just now.  I'll try to create test cases.  I can create test cases for VE like checking live-in problems described in #78942 .  However, such tests work only on VE environment.  So, I think it's better to have different tests something like to check the output of TableGen directly.  I'll try to create such one.

https://github.com/llvm/llvm-project/pull/79462