x86::tbm::bextri triggers an LLVM codegen error

[gnzlbg]

It is the only instruction not implemented from the TBM feature set.

[gnzlbg]

@alexcrichton @BurntSushi the root of the issue is that tbm::bextri takes an argument (u32/u64) that must be immediate. Lucky for us, only the first 16 bits of this value matter. Unlucky for us, using a constify_imm16!, with a match statement containing 65534 arms, makes the stdsimd crate take between forever and never to compile 😆

Thoughts?

[alexcrichton]

I'd probably just leave this open for now, I'm not sure how we'd fix it :(