Constify all x86 rustc_args_required_const intrinsics

[bjorn3]

Fixes https://github.com/bjorn3/rustc_codegen_cranelift/issues/669

I am not quite sure about how some intrinsics should handle imm8 values > 255. Where there was no masking before, I have used saturating through constify_imm8.

[rust-highfive]

r? @gnzlbg

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[bjorn3]

cc @eddyb (https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/stable.20subset.20of.20const.20generics/near/203882415)

[Amanieu]

IMO we should panic on out-of-range immediates, with the intention of turning those into compile-time errors once we have enough support for const generics (and #[rustc_args_required_const] becomes syntax sugar for const generics).

[bjorn3]

All other #[rustc_args_required_const] intrinsics either mask or saturate the immediate.

[Amanieu]

I reviewed the existing code and it seems that they all use masking via the constify_* macros. Let's just stick to that for now.