auto merge of #18080 : veddan/rust/assume, r=thestinger

Adds an `assume` intrinsic that gets translated to llvm.assume. It is
used on a boolean expression and allows the optimizer to assume that
the expression is true.

This implements #18051.

src/test/run-pass/intrinsic-assume.rs

@@ -0,0 +1,25 @@
+// Copyright 2014 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.
+
+use std::intrinsics::assume;
+
+unsafe fn f(x: i32) -> i32 {
+    assume(x == 34);
+    match x {
+        34 => 42,
+        _  => 30
+    }
+}
+
+fn main() {
+    let x = unsafe { f(34) };
+    assert_eq!(x, 42);
+}
+