[AArch64] Extend redundant copy elimination pass to handle non-zero stores.

This patch extends the current functionality of the AArch64 redundant copy
elimination pass to handle non-zero cases such as:

BB#0:
  cmp x0, #1
  b.eq .LBB0_1
.LBB0_1:
  orr x0, xzr, #0x1  ; <-- redundant copy; x0 known to hold #1.

Differential Revision: https://reviews.llvm.org/D29344

llvm-svn: 296809