Add an explicit test for let _ = *ptr

Author: saethlin

tests/ui/mir/alignment/place_without_read.rs

@@ -0,0 +1,9 @@
+// run-pass
+// compile-flags: -C debug-assertions
+
+fn main() {
+    let ptr = 1 as *const u16;
+    unsafe {
+        let _ = *ptr;
+    }
+}