Fix prefix emission order to emit REX immediately before the opcode (SDM vol2,
2.1, Figure 2-1). According to SDM vol2 2.2.1, "Other placements are ignored".
This fix has a side effect of outputting segment override prefix in a different
order than previously (benign).
Follow-up to https://reviews.llvm.org/D120592
Reviewed By: skan, craig.topper
Differential Revision: https://reviews.llvm.org/D120871
