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

[Jay Foad via llvm-commits]

jayfoad wrote:

This gives a measurable X86 -O3 compile time speedup: https://llvm-compile-time-tracker.com/compare.php?from=594d57e07a92e3a2cefb262114db2608989f874d&to=aa8fca82b83239cfb186801238c282bd01fc48c2&stat=instructions:u

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