[ConstantHoisting] avoid hang/crash from unreachable blocks (PR40930)

[ConstantHoisting] avoid hang/crash from unreachable blocks (PR40930)

I'm not too familiar with this pass, so there might be a better
solution, but this appears to fix the degenerate:
PR40930
PR40931
PR40932
PR40934
...without affecting any real-world code.

As we've seen in several other passes, when we have unreachable blocks,
they can contain semi-bogus IR and/or cause unexpected conditions. We
would not typically expect these patterns to make it this far, but we
have to guard against them anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@355337 91177308-0d34-0410-b5e6-96231b3b80d8