RegisterCoalescer: Remove phi-only subranges when erasing identity copies

Undef subranges are not present in the live range values, except when
they cross block boundaries. In this situation, a identity copy is
inside a loop, and one of the lanes is undefined. It only appears
alive inside the loop due to the copy. Once the copy was erased, it
would leave behind a segment inside the loop body with no
corresponding def anywhere in the program.

When RenameIndependentSubregs processed this dummy interval, it would
introduce a "Multiple connected components in live interval" verifier
error when IMPLICIT_DEFs were added to the other two blocks. I believe
there is a missing verifier check for this type of dummy interval.

I have found additional cases from the same fundamental problem in
other areas I haven't managed to fix yet (e.g. the commented out
prune_subrange_phi_value_* cases).