Teach RegisterCoalescer to handle symmetric sub-register copies.

It is possible to coalesce two overlapping registers to a common
super-register that it larger than both of the original registers.

The important difference is that it may be necessary to rewrite DstReg
operands as well as SrcReg operands because the sub-register index has
changed.

This behavior is still disabled by CoalescerPair.

llvm-svn: 156869