[AArch64] Enable merging of adjacent zero stores for all subtargets.

This optimization merges adjacent zero stores into a wider store.

e.g.,

strh wzr, [x0]
strh wzr, [x0, #2]
; becomes
str wzr, [x0]

e.g.,

str wzr, [x0]
str wzr, [x0, #4]
; becomes
str xzr, [x0]

Previously, this was only enabled for Kryo and Cortex-A57.

Differential Revision: https://reviews.llvm.org/D26396

llvm-svn: 286592