Add test showing how byte slices are printed in MIR

src/test/mir-opt/byte_slice.rs

@@ -0,0 +1,15 @@
+// compile-flags: -Z mir-opt-level=0
+
+fn main() {
+    let x = b"foo";
+    let y = [5u8, b'x'];
+}
+
+// END RUST SOURCE
+// START rustc.main.EraseRegions.after.mir
+// ...
+// _1 = const Scalar(Ptr(Pointer { alloc_id: AllocId(0), offset: Size { raw: 0 }, tag: () })) : &[u8; 3];
+// ...
+// _2 = [const 5u8, const 120u8];
+// ...
+// END rustc.main.EraseRegions.after.mir