AVX512-VP2INTERSECT implementations

[vertexclique]

Currently, these implementations aren't finished. Link representations for N x i1 for llvm needs to be incorporated into the compiler.
The best way to do the i1 linking is using the proc macro attribute on extern to make linking explicit to i1 representations.

Note that bool is still i8 representation from the rust's side with zero extend. That hasn't been solved for linking to llvm.

Relates to #989 and rust-lang/rust#81552

[rust-highfive]

r? @Amanieu

(rust-highfive has picked a reviewer for you, use r? to override)

[vertexclique]

we have talked with @Amanieu about it. This can wait until we land the i1 linking in the compiler.

[Gab-Menezes]

Hey @Amanieu, first I would like to thank you for your work.

Any plans on continue development to support this instrinsics ?
I tried using this instructions with inline asm, but without much success, since they "clobber" an undermined register (the clobbed kreg depends on the the first operand, since the masks are written in a even/odd pair). AFAIK to use this with inline asm the first operand needs to be fixed and can't be generated dynamically by the compiler, since if the compiler choose the register kX the register kX+1 will also be written, but you can't specify kX+1 as an output.
I would like to help if needed, but I have never contributed to stdarch, so if you can point me to some resources.

[sayantn]

Once rust-lang/rust#140763 lands, we will be able to finally implement this. We will also be able to remove uses of many deprecated intrinsics.