[llvm] [TableGen] Fix regunit superset calculation (PR #81850)

[Jay Foad via llvm-commits]

jayfoad wrote:

> This is still a draft because I'd like to understand why it affected X86 codegen. I assume it is due to some difference in the pressure sets affecting the scheduler.

Confirmed. I looked at a couple of cases and it was due to different scheduler heuristics that depend on pressure sets.

https://github.com/llvm/llvm-project/pull/81850