opflags: Separate vector registers into low-16 and high-16

opflags: Separate vector registers into low-16 and high-16

Since only EVEX supports all 32 vector registers encoding for now,
VEX/REX encoded instructions should not take high-16 registers as operands.

This filtering had been done using instruction flag so far, but
using the opflags makes more sense.

[XYZ]MMREG operands used for non-EVEX instructions are automatically
converted to [XYZ]MM_L16 in insns.pl

Signed-off-by: Jin Kyu Song <jin.kyu.song@intel.com>