There is a debug-build-only verification that is done for
alloc_pack_metadata instructions that checks that there exist paired
dealloc_pack_metadata instructions which will be keyed off of to clean
up the on-stack variadic metadata packs corresponding to (the
instruction after) the alloc_pack_metadata.
StackNesting omits the deallocation instruction (as does
PackMetadataMarkerInserter) if it would be created in a dead end block.
If all blocks in the dominance frontier of the alloc_pack_metadata
instruction are dead-end blocks, then the verification will incorrectly
fail. It should not fail because it is not necessary to clean up the
on-stack pack metadata (or any other stack allocations) in such a case.
If all such blocks are dead-end blocks, however, the
alloc_pack_metadata's block itself is a dead-end block as well. So
during the verification, check whether the alloc_pack_metadata occurs in
a dead-end block and do not fail verification if it does.
rdar://125265980
