[BOLT] In shrinkwrap, do not split prefix/instr

Summary:
When placing restore instructions in the shrink wrapping pass,
we typically put them right before the last instruction of a block at
the dominance frontier. If this instruction happened to have a prefix,
because the MC lib separates prefix into separate MCInsts, we would
accidentally put a load between a prefix and another instruction. Fix
this.

(cherry picked from FBD24295324)