[x86] fix allowsMisalignedMemoryAccesses() for 8-byte and smaller accesses

This is a continuation of the fix from:
http://reviews.llvm.org/D10662

and discussion in:
http://reviews.llvm.org/D12154

Here, we distinguish slow unaligned SSE (128-bit) accesses from slow unaligned
scalar (64-bit and under) accesses. Other lowering (eg, getOptimalMemOpType) 
assumes that unaligned scalar accesses are always ok, so this changes 
allowsMisalignedMemoryAccesses() to match that behavior.

Differential Revision: http://reviews.llvm.org/D12543

llvm-svn: 246658