[AArch64] Redundant Copy Elimination - remove more zero copies.

This patch removes unnecessary zero copies in BBs that are targets of b.eq/b.ne
and we know the result of the compare instruction is zero.  For example,

BB#0:
  subs w0, w1, w2
  str w0, [x1]
  b.ne .LBB0_2
BB#1:
  mov w0, wzr  ; <-- redundant
  str w0, [x2]
.LBB0_2

Differential Revision: https://reviews.llvm.org/D35075

llvm-svn: 308849