rename "slow-unaligned-mem-under-32" to slow-unaligned-mem-16" (NFCI)

This is a follow-on suggested by:
http://reviews.llvm.org/D12154 ( http://reviews.llvm.org/rL245729 )
http://reviews.llvm.org/D10662 ( http://reviews.llvm.org/rL245075 )

This makes the attribute name match most of the existing lowering logic
and regression test expectations.

But the current use of this attribute is inconsistent; see the FIXME
comment for "allowsMisalignedMemoryAccesses()". That change will
result in functional changes and should be coming soon.

llvm-svn: 246585