iflags: Add IF_EVEX for checking {evex} availability

iflags: Add IF_EVEX for checking {evex} availability

For checking the availability of {evex} prefix, AVX512 iflag
has been used. But this is a flag for an instruction set
not for an encoding scheme. And there are some AVX512 instructions
encoded with VEX prefix.

So a new instruction flag (IF_EVEX) is added for the instructions
which are actually encoded with EVEX prefix.

This flag is automatically added by insns.pl, so no need to add manually
in insns.dat.

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