[CodeGen] Fix multiple connected component issue in rematerializer (#186674)
This fixes a rematerializer issue wherein re-creating the interval of a
non-rematerializable super-register defined over multiple MIs, some of
which defining entirely dead sub-registers, could cause a crash when
changing the order of sub-definitions (for example during scheduling)
because the re-created interval could end up with multiple connected
components, which is illegal. The solution is to split separate
components of the interval in such cases. The added unit test crashes
without that added behavior.