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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246658 91177308-0d34-0410-b5e6-96231b3b80d8