[AArch64] Handle missing store pair opportunity

Summary:
This change will handle missing store pair opportunity where the first store
instruction stores zero followed by the non-zero store. For example, this change
will convert :

  str wzr, [x8]
  str w1, [x8, #4]
into:
  stp wzr, w1, [x8]

Reviewers: jmolloy, t.p.northover, mcrosier

Subscribers: flyingforyou, aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D18570

llvm-svn: 265021