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

[Jay Foad via llvm-commits]

jayfoad wrote:

> before closing this PR ( as its alternative fix of yours has been merged), could you comment on additional tests (in this PR) that may be needed as, your fix also have the similar tests. Then only will it makes sense to add it extra tests as new PR!

Your test `DisjointLeafAliasSubregs.td` looks identical to the B/Blo/Bhi test case that I added in #139526.

Your test `DisjointNonLeafAliasSubregs.td` looks slightly more complex than any of my test cases, so you could perhaps add that as an extra test case if you want. I don't really mind either way. If you_do_ add it, please just add it as an extra case in the existing file `SubRegsAndAliases.td`.

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