x86: Require AVX support for bit compress/expand

[Spencer-Comin]

AVX support is required to generate VEX prefix for pext and pdep instructions.

[BradleyWood]

It's not well documented if VEX encoded BMI2 instructions can run when AVX is not supported, but either way, binary encoding will not behave correctly in this case.

I also found a similar issue with FMA opcodes used in OpenJ9, which I will fix.

[0xdaryl]

Sorry, what is the motivation for this PR? Is it because our current binary encoder won't let you encode VEX instructions unless AVX is supported, or is it just defensive programming?

My understanding is that from the processor's perspective, AVX is not a necessary pre-req for BMI2.

[Spencer-Comin]

Our current binary encoder won't let you encode VEX instructions unless AVX is supported. There aren't any physical processors that I'm aware of with BMI2 but not AVX. As far as BIOS/hypervisor disabling AVX while requiring BMI2, theoretically this could happen, but I imagine that happens rarely enough (never?) that we won't run into a situation where not having bitwise compress/expand support even when it's technically supported becomes a problem.