[llvm] Reland [SimplifyCFG] Delete the unnecessary range check for small mask operation (PR #70542)

[via llvm-commits]

zmodem wrote:

> > Thanks! The fix looks right to me. It would be good to have a test which covers the case where HasDefaultResults is false though.
> 
> hi @zmodem, the new added `switch_lookup_with_small_i1_unreachable` already matched the case where `HasDefaultResults is false`, so I can rename it to **switch_lookup_with_small_i1_default_no_results** if its name is easy to confuse ? (HasDefaultResults is false before the fix, so an error is reported when DefaultResults is used to fill in table hole.)

I think I've also confused myself about `HasDefaultResults` and `DefaultIsReachable`.

`!DefaultIsReachable` implies `!HasDefaultResults`

so yes, the current test covers where we asserted before, but there are other situations where `DefaultIsReachable` can be false, such as when the default case does not result in a constant. I think it would be good if the test covered that too.

https://github.com/llvm/llvm-project/pull/70542