Auto merge of #1720 - RalfJung:rustup, r=RalfJung

rustup

Prevent MIR optimizations from removing this read.

rust-version

@@ -1 +1 @@
-e7c23ab933ebc1f205c3b59f4ebc85d40f67d404
+a4e595db8f12f9ee926256745d757004b850703f

tests/compile-fail/transmute-pair-uninit.rs

@@ -17,6 +17,6 @@ fn main() {
         assert_eq!(byte, 0);
     }
     let v = unsafe { *z.offset(first_undef) };
-    if v == 0 {}
+    if v == 0 { println!("it is zero"); }
     //~^ ERROR this operation requires initialized memory
 }