RegisterCoalescer: Prune undef subranges from copy pairs in loops

If we had a pair of copies inside a loop which introduced new liveness
to a subregister which was undef before the loop, we would have a
dummy phi-only segment remaining across the loop body. Later, this
false segment would confuse RenameIndependentSubregs causing it to
introduce IMPLICIT_DEFs with broken value numbering.

It seems always adding the lanes to ShrinkMask is OK, so any
conditions should be purely a compile time filter.