Support AVX2 and BMI2 instructions

[ihnorton]

Reminder to revert the temporary fix for #4362.

Related LLVM bug: http://llvm.org/bugs/show_bug.cgi?id=17422

This would likely be subsumed by switching to MCJIT.

See also a similar bug in AVX support, resolved by backporting some functionality from MCJIT.

[Keno]

The LLVM folks have been actively trying to push people off the legacy JIT, but I'm not sure we can support MCJIT before they implement multi-module support. It does seem like that is underway at the moment though, so maybe just stay put until that is resolved.

[blakejohnson]

It may be worth keeping this open until we have removed the cases where we disabled AVX2 and BMI2.

[JeffBezanson]

Bump. Can we remove the workaround now?

[vtjnash]

it's been conditional on the llvm version for a long time, I just don't feel like doing blame to check which commit did it

[kmsquire]

Just for documentation: workaround is here: https://github.com/JuliaLang/julia/blob/master/src/codegen.cpp#L5635-L5639

I don't see a condition on LLVM version, but maybe I missed it...?