Support #[rustc_args_required_const(...)] for simd intrinsics

[bjorn3]

For example:

// https://github.com/rust-lang/stdarch/blob/ef6b0690192f1cfc753af698695c2ecde0c7b991/crates/core_arch/src/x86_64/avx2.rs#L31
#[rustc_args_required_const(1)]
pub unsafe fn _mm256_extract_epi64(a: __m256i, imm8: i32) -> i64 {
    let imm8 = (imm8 & 3) as u32;
    simd_extract(a.as_i64x4(), imm8)
}

Calling these kind of intrinsics will currently panic with a message like: simd_extract* idx not const: "move _8".

[bjorn3]

Using -Zmir-opt-level=3 doesn't help. While the function gets inlined, there is no constant propagation performed to get rid of the (imm & 3) as u32 part.

[bjorn3]

I made these functions panic at runtime for now.

[bjorn3]

Fixing rust-lang/rust#70271 would fix this.

[bjorn3]

There has been a discussion about this on zulip starting at https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/change.20rustc_args_required_const.20to.20per.20arg.20attribute/near/191390244.