rustc: handle allocas and LoadRangeAsserts in unreachable blocks correctly.

An alloca in an unreachable block would shortcircuit with Undef, but with type
`Type`, rather than type `*Type` (i.e. a plain value, not a pointer) but it is
expected to return a pointer into the stack, leading to confusion and LLVM
asserts later.

Similarly, attaching the range metadata to a Load in an unreachable block
makes LLVM unhappy, since the Load returns Undef.

Fixes #7344.

src/test/run-pass/issue-7344.rs

@@ -0,0 +1,28 @@
+// Copyright 2013 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.
+
+#[allow(unreachable_code)];
+
+fn foo() -> bool { false }
+
+fn bar() {
+    return;
+    !foo();
+}
+
+fn baz() {
+    return;
+    if "" == "" {}
+}
+
+fn main() {
+    bar();
+    baz();
+}