[AArch64] Add custom store lowering for 256 bit non-temporal stores.

Currently we fail to lower non-termporal stores for 256+ bit vectors
to STNPQ, because type legalization will split them up to 128 bit stores
and because there are no single non-temporal stores, creating STPNQ
in the Load/Store optimizer would be quite tricky.

This patch adds custom lowering for 256 bit non-temporal vector stores
to improve the generated code.

Reviewers: dmgreen, samparker, t.p.northover, ab

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D72919