[ARM] Don't split trunc stores that can be better handled as VMOVN

We deliberately split stores of the form
store(truncate(larger-than-legal-type)) into two stores, allowing each
store to perform part of the truncate for free.

There are times however where it makes more sense to use VMOVN to
de-interlace the results back into a single vector, and store that in
one go. This adds a check for that situation, not splitting the store if
it looks like a VMOVN can be more useful.

Differential Revision: https://reviews.llvm.org/D76511

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