Reapply [CHR] Fix up phi nodes with unreachable predecessors (PR64594)

Relative to the previous attempt, this also adjusts RegionInfo
verification to allow unreachable predecessors.

-----

If a block in the CHR region has an unreachable predecessor, then
there will be no edge from that predecessor to the newly cloned
block. However, a phi node entry for it will be left behind. Make
sure that these incoming blocks get dropped as well.

Fixes https://github.com/llvm/llvm-project/issues/64594.

Differential Revision: https://reviews.llvm.org/D157621