[AArch64] Decouple zero store promotion from narrow ld merge. NFC.

Summary: This change refactors to decouple the zero store promotion from the narrow ld merge and add a flag (enable-narrow-ld-merge=true) to control the narrow ld merge optimization.

Reviewers: jmolloy, t.p.northover, mcrosier

Subscribers: aemerson, rengolin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19885

llvm-svn: 268744