[SimplifyCFG] simplifyUnreachable(): erase instructions iff they are...

[SimplifyCFG] simplifyUnreachable(): erase instructions iff they are guaranteed to transfer execution to unreachable

This replaces the current ad-hoc implementation,
by syncing the code from InstCombine's implementation in `InstCombinerImpl::visitUnreachableInst()`,
with one exception that here in SimplifyCFG we are allowed to remove EH instructions.

Effectively, this now allows SimplifyCFG to remove calls (iff they won't throw and will return),
arithmetic/logic operations, etc.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D105374