Automatically use VEX-encoding in AVX/AVX2/XOP/FMA3/FMA4
functions for all instructions that exists in a VEX-encoded
version.

This change makes it easier to extend existing code to use AVX2.

Also add support for AVX emulation of a few instructions that
were missing before.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>