Auto merge of #49141 - gnzlbg:simd_select, r=alexcrichton

adds simd_select intrinsic

The select SIMD intrinsic is used to select elements from two SIMD vectors using a mask:

```rust
let mask = b8x4::new(true, false, false, true);
let a = f32x4::new(1., 2., 3., 4.);
let b = f32x4::new(5., 6., 7., 8.);
assert_eq!(simd_select(mask, a, b), f32x4::new(1., 6., 7., 4.));
```

The number of lanes between the mask and the vectors must match, but the vector width of the mask does not need to match that of the vectors. The mask is required to be a vector of signed integers.

Note: this intrinsic will be exposed via `std::simd`'s vector masks - users are not expected to use it directly.

src/test/compile-fail/simd-intrinsic-generic-select.rs

@@ -0,0 +1,56 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that the simd_select intrinsic produces ok-ish error
+// messages when misused.
+
+#![feature(repr_simd, platform_intrinsics)]
+#![allow(non_camel_case_types)]
+
+#[repr(simd)]
+#[derive(Copy, Clone)]
+pub struct f32x4(pub f32, pub f32, pub f32, pub f32);
+
+#[repr(simd)]
+#[derive(Copy, Clone)]
+pub struct u32x4(pub u32, pub u32, pub u32, pub u32);
+
+#[repr(simd)]
+#[derive(Copy, Clone, PartialEq)]
+struct b8x4(pub i8, pub i8, pub i8, pub i8);
+
+#[repr(simd)]
+#[derive(Copy, Clone, PartialEq)]
+struct b8x8(pub i8, pub i8, pub i8, pub i8,
+            pub i8, pub i8, pub i8, pub i8);
+
+extern "platform-intrinsic" {
+    fn simd_select<T, U>(x: T, a: U, b: U) -> U;
+}
+
+fn main() {
+    let m4 = b8x4(0, 0, 0, 0);
+    let m8 = b8x8(0, 0, 0, 0, 0, 0, 0, 0);
+    let x = u32x4(0, 0, 0, 0);
+    let z = f32x4(0.0, 0.0, 0.0, 0.0);
+
+    unsafe {
+        simd_select(m4, x, x);
+
+        simd_select(m8, x, x);
+        //~^ ERROR mismatched lengths: mask length `8` != other vector length `4`
+
+        simd_select(x, x, x);
+        //~^ ERROR mask element type is `u32`, expected `i_`
+
+        simd_select(z, z, z);
+        //~^ ERROR mask element type is `f32`, expected `i_`
+    }
+}