[AArch64]Merge narrow zero stores to a wider store

[AArch64]Merge narrow zero stores to a wider store

This change merges adjacent zero stores into a wider single store.
For example :
  strh wzr, [x0]
  strh wzr, [x0, #2]
becomes
  str wzr, [x0]

This will fix PR25410.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253711 91177308-0d34-0410-b5e6-96231b3b80d8