add aliases for movs between seg registers and mem. There are multiple

add aliases for movs between seg registers and mem.  There are multiple
different forms of this instruction (movw/movl/movq) which we reported
as being ambiguous.  Since they all do the same thing, gas just picks the
one with the shortest encoding.  Follow its lead here.

This implements rdar://8208615

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@118362 91177308-0d34-0410-b5e6-96231b3b80d8