[AArch64] Merge two adjacent str WZR into str XZR

Summary:
This change merges adjacent 32 bit zero stores into a 64 bit zero store.
e.g.,
  str wzr, [x0]
  str wzr, [x0, #4]
becomes
  str xzr, [x0]

Therefore, four adjacent 32 bit zero stores will be a single stp.
e.g.,
  str wzr, [x0]
  str wzr, [x0, #4]
  str wzr, [x0, #8]
  str wzr, [x0, #12]
becomes
  stp xzr, xzr, [x0]

Reviewers: mcrosier, jmolloy, gberry, t.p.northover

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D16933

llvm-svn: 260682