AArch64: avoid splitting vector truncating stores.

We have code to split vector splats (of zero and non-zero) for performance
reasons, but it ignores the fact that a store might be truncating.

Actually, truncating stores are formed for vNi8 and vNi16 types. Since the
truncation is from a legal type, the size of the store is always <= 64-bits and
so they don't actually benefit from being split up anyway, so this patch just
disables that transformation.

llvm-svn: 350620