[llvm] [CodeGen] For ad hoc aliasing, additional regUnits are needed to fix lanemask representation (PR #139206)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

This could do with a test. See `test/TableGen/ArtificialSubregs.td` for example. If you extend the `-register-info-debug` to print regunits as well as subregs, then you can test this patch in the same kind of way.

https://github.com/llvm/llvm-project/pull/139206