rustc: Add support for some more x86 SIMD ops

This commit adds compiler support for two basic operations needed for binding
SIMD on x86 platforms:

* First, a `nontemporal_store` intrinsic was added for the `_mm_stream_ps`, seen
  in rust-lang-nursery/stdsimd#114. This was relatively straightforward and is
  quite similar to the volatile store intrinsic.

* Next, and much more intrusively, a new type to the backend was added. The
  `x86_mmx` type is used in LLVM for a 64-bit vector register and is used in
  various intrinsics like `_mm_abs_pi8` as seen in rust-lang-nursery/stdsimd#74.
  This new type was added as a new layout option as well as having support added
  to the trans backend. The type is enabled with the `#[repr(x86_mmx)]`
  attribute which is intended to just be an implementation detail of SIMD in
  Rust.

I'm not 100% certain about how the `x86_mmx` type was added, so any extra eyes
or thoughts on that would be greatly appreciated!

src/test/codegen/nontemporal.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 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.
+
+// compile-flags: -O
+
+#![feature(core_intrinsics)]
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn a(a: &mut u32, b: u32) {
+    // CHECK-LABEL: define void @a
+    // CHECK: store i32 %b, i32* %a, align 4, !nontemporal
+    unsafe {
+        std::intrinsics::nontemporal_store(a, b);
+    }
+}

src/test/codegen/x86_mmx.rs

@@ -0,0 +1,30 @@
+// Copyright 2017 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.
+
+// ignore-arm
+// ignore-aarch64
+// ignore-emscripten
+// compile-flags: -O
+
+#![feature(repr_simd)]
+#![crate_type="lib"]
+
+#[repr(simd)]
+#[derive(Clone, Copy)]
+pub struct i8x8(u64);
+
+#[no_mangle]
+pub fn a(a: &mut i8x8, b: i8x8) -> i8x8 {
+    // CHECK-LABEL: define x86_mmx @a(x86_mmx*{{.*}}, x86_mmx{{.*}})
+    // CHECK: store x86_mmx %b, x86_mmx* %a
+    // CHECK: ret x86_mmx %b
+    *a = b;
+    return b
+}