[llvm] [TableGen] Simplify generated code for isSubclass (PR #117351)

[Jay Foad via llvm-commits]

jayfoad wrote:

> IIUC we could have two nested switches (constant time lookup), but now we are using binary search (logarithmic time lookup). Will this cause any time related regressions?

Good question. I tried measuring this with `check-llvm-mc` but the run-to-run variation was pretty high. I'll see if I can detect any difference in a single execution of `llvm-mc`.

https://github.com/llvm/llvm-project/pull/117351