[CodeGen,AArch64] Fix up warnings in splitStores

The code for trying to split up stores is designed for NEON vectors,
where we support arbitrary alignments. It's an optimisation designed
to improve performance by using smaller, aligned stores. However,
we currently only support 16 byte alignments for SVE vectors anyway
so we may as well bail out early.

This change fixes up remaining warnings in a couple of tests:

  CodeGen/AArch64/sve-callbyref-notailcall.ll
  CodeGen/AArch64/sve-calling-convention-byref.ll

Differential Revision: https://reviews.llvm.org/D80720