[AArch64] Add pass to remove redundant copy after RA

Summary:
This change will add a pass to remove unnecessary zero copies in target blocks
of cbz/cbnz instructions. E.g., the copy instruction in the code below can be
removed because the cbz jumps to BB1 when x0 is zero :
  BB0:
    cbz x0, .BB1
  BB1:
    mov x0, xzr

Jun

Reviewers: gberry, jmolloy, HaoLiu, MatzeB, mcrosier

Subscribers: mcrosier, mssimpso, haicheng, bmakam, llvm-commits, aemerson, rengolin

Differential Revision: http://reviews.llvm.org/D16203

llvm-svn: 261004