Rework segment prefix emission code to handle segments

Rework segment prefix emission code to handle segments
in memory operands at the same type as hard coded segments.
This fixes problems where we'd emit the segment override after
the REX prefix on instructions like:
mov %gs:(%rdi), %rax

This fixes rdar://8127102.  I have several cleanup patches coming
next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107917 91177308-0d34-0410-b5e6-96231b3b80d8