[ubsan] Omit return value check when return block is unreachable

If the return block is unreachable, clang removes it in
CodeGenFunction::FinishFunction(). This removal can leave dangling
references to values defined in the return block if the return block has
successors, which it /would/ if UBSan's return value check is emitted.

In this case, as the UBSan check wouldn't be reachable, it's better to
simply not emit it.

rdar://59196131