Make sure that casting a ptr-integer down to u8 makes it unusable

tests/compile-fail/ptr_int_cast.rs

@@ -0,0 +1,7 @@
+fn main() {
+    let x = &1;
+    // Casting down to u8 and back up to a pointer loses too much precision; this must not work.
+    let x = x as *const i32 as u8;
+    let x = x as *const i32; //~ ERROR: a raw memory access tried to access part of a pointer value as raw bytes
+    let _ = unsafe { *x };
+}