Be more forgiving when calculating alias interference for physreg coalescing.

It is OK for an alias live range to overlap if there is a copy to or from the
physical register. CoalescerPair can work out if the copy is coalescable
independently of the alias.

This means that we can join with the actual destination interval instead of
using the getOrigDstReg() hack. It is no longer necessary to merge clobber
ranges into subregisters.

llvm-svn: 107695