[NFCI][InstCombine] visitUnreachableInst(): iteratively erase instructions leading to unreachable

In the original review D87149 it was mentioned that this approach was tried,
and it lead to infinite combine loops, but i'm not seeing anything like that now,
neither in the `check-llvm`, nor on some codebases i tried.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D105339