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

[Quentin Colombet via llvm-commits]

qcolombet wrote:

Do you have an example of a diff of the `.inc` file generated before and after this patch?

I found it fascinating (in a good way) that changing how we compute these improves compile time, so I'm curious to see a diff firsthand.

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