[X86] Improve code size on X86 segment moves

Moves of a value to a segment register from a 16-bit register is
equivalent to one from it's corresponding 32-bit register. Match gas's
behavior and rewrite instructions to the shorter of equivalent forms.

Reviewers: rnk, ab

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23166

llvm-svn: 278031