[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.

This is a recommit of d9d65527,
which i immediately reverted because i have messed up something
during branch switch, and 597ccc92
accidentally ended up being pushed, which was very much not the intention.

Reviewed By: spatel

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