Constification part 5.
Constify ppXXXInstr, getRegUsage_XXXInstr, isMove_XXXInstr, emit_XXXInstr,
and iselSB_XXX.


git-svn-id: svn://svn.valgrind.org/vex@2970 8f6e269a-dfd6-0310-a8e1-e2731360e62c