[RegisterCoalescing] Recommit the patch "Remove partial redundent copy".

In r292621, the recommit fixes a bug related with live interval update
after the partial redundent copy is moved.

This recommit solves an additional bug related to the lack of update of
subranges.

The original patch is to solve the performance problem described in
PR27827. Register coalescing sometimes cannot remove a copy because of
interference. But if we can find a reverse copy in one of the predecessor
block of the copy, the copy is partially redundent and we may remove the
copy partially by moving it to the predecessor block without the
reverse copy.

Differential Revision: https://reviews.llvm.org/D28585

Re-apply r292621

Revert "Revert rL292621. Caused some internal build bot failures in apple."

This reverts commit r292984.

Original patch: Wei Mi <wmi@google.com>
Subrange fix: Mostly Matthias Braun <matze@braunis.de>

llvm-svn: 293353