[RegisterCoalescing] Remove partial redundent copy.

The 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

llvm-svn: 292292