[SimplifyCFG] avoid crash on degenerate loop

The problematic code pattern in the test is based on:
https://llvm.org/PR50638

If the IfCond is itself the phi that we are trying to remove,
then the loop around line 2835 can end up with something like:
%cmp = select i1 %cmp, i1 false, i1 true

That can then lead to a use-after-free and assert (although
I'm still not seeing that locally in my release + asserts build).

I think this can only happen with unreachable code.

Differential Revision: https://reviews.llvm.org/D104063