[AArch64] Improve single vector lane unscaled stores

When storing the 0th lane of a vector, use a simpler and usually more
efficient scalar store instead.  In this case, also using the unscaled
offset.

Differential revision: https://reviews.llvm.org/D46762

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332394 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed