maskStore improvements

Little follow-up to !233 (merged) after I thought about it again.

• fix the aligned version (it was using maskStore in some instruction sets and maskStoreA in others)
• make sure the test case is incommensurate with the vector width (previously it couldn't distinguish store from storeMask on 128-bit vector instruction sets)
• implement a fallback for instruction sets that don't support it natively (turns out this is really easy using a load-blend-store combination)