Rollup merge of rust-lang#128731 - RalfJung:simd-shuffle-vector, r=workingjubilee

simd_shuffle intrinsic: allow argument to be passed as vector

See rust-lang#128738 for context.

I'd like to get rid of [this hack](https://github.com/rust-lang/rust/blob/6c0b89dfac65be9a5be12f938f23098ebc36c635/compiler/rustc_codegen_ssa/src/mir/block.rs#L922-L935). rust-lang#128537 almost lets us do that since constant SIMD vectors will then be passed as immediate arguments. However, simd_shuffle for some reason actually takes an *array* as argument, not a vector, so the hack is still required to ensure that the array becomes an immediate (which then later stages of codegen convert into a vector, as that's what LLVM needs).

This PR prepares simd_shuffle to also support a vector as the `idx` argument. Once this lands, stdarch can hopefully be updated to pass `idx` as a vector, and then support for arrays can be removed, which finally lets us get rid of that hack.

Author: tgross35

Diff for: core/src/intrinsics/simd.rs

@@ -232,7 +232,7 @@ extern "rust-intrinsic" {
     ///
     /// `T` must be a vector.
     ///
-    /// `U` must be a **const** array of `i32`s. This means it must either refer to a named
+    /// `U` must be a **const** array or vector of `u32`s. This means it must either refer to a named
     /// const or be given as an inline const expression (`const { ... }`).
     ///
     /// `V` must be a vector with the same element type as `T` and the same length as `U`.