[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 d9d65527c289fb27a9f92f150723bbb3c58e413f,
which i immediately reverted because i have messed up something
during branch switch, and 597ccc92ce4b0f90883406d1f78d9d776f602804
accidentally ended up being pushed, which was very much not the intention.
Reviewed By: spatel
Differential Revision: https://reviews.llvm.org/D105339