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

[Kazushi Marukawa via llvm-commits]

kaz7 wrote:

@jayfoad ,

I made a test in https://github.com/kaz7/llvm-project/tree/computeregunitlanemasks-test.  I tried to push it to your fork, but it won't work.  Can you cherry-pick my patch?  This test work fine with a tablegen after this PR.  And, it causes errors if someone try to calculate lanemask using intersection.

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