[Inline][Cloning] Reliably remove unreachable blocks during cloning (PR53206)

The pruning cloner already tries to remove unreachable blocks. The
original cloning process will simplify instructions and constant
terminators, and only clone blocks that are reachable at that point.
However, phi nodes can only be simplified after everything has been
cloned. For that reason, additional blocks may become unreachable
after phi simplification.

The code does try to handle this as well, but only removes blocks
that don't have predecessors. It misses unreachable cycles. This
can cause issues if SEH exception handling code is part of an
unreachable cycle, as the inliner is not prepared to deal with that.

This patch instead performs an explicit scan for reachable blocks,
and drops everything else.

Fixes https://github.com/llvm/llvm-project/issues/53206.

Differential Revision: https://reviews.llvm.org/D118449