[AArch64TTI] AArch64 supports NT vector stores through STNP.

This patch adds a custom implementation of isLegalNTStore to AArch64TTI
that supports vector types that can be directly stored by STNP. Note
that the implementation may not catch all valid cases (e.g. because the
vector is a multiple of 256 and could be broken down to multiple valid 256 bit
stores), but it is good enough for LV to vectorize loops with NT stores,
as LV only passes in a vector with 2 elements to check. LV seems to also
be the only user of isLegalNTStore.

We should also do the same for NT loads, but before that we need to
ensure that we properly lower LDNP of vectors, similar to D72919.

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

Reviewed By: dmgreen

Differential Revision: https://reviews.llvm.org/D73158