[X86] Avoid unecessary opsize byte in segment move to memory

Summary:

Segment moves to memory are always 16-bit. Remove invalid 32 and 64
bit variants.

Fixes PR34478.

Reviewers: rnk, craig.topper

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D39847

llvm-svn: 318678