x86inc: activate REP_RET automatically
Now RET checks whether it immediately follows a branch, so the programmer dosen't have to keep track of that condition.
REP_RET is still needed manually when it's a branch target, but that's much rarer.
The implementation involves lots of spurious labels, but that's ok because we strip them.
x86inc: Use VEX-encoded instructions in AVX functions
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.
AVX2/FMA3 version of mbtree_propagate
First AVX2 function for testing.
Bump yasm version to 1.2.0 for AVX2 support.
x86inc: Drop tzcnt workaround

It is no longer needed now that we've bumped the version requirement of yasm to 1.2.0.
x86inc: sync with libav