make sure we catch alignment problems even with intrptrcast

tests/compile-fail/intptrcast_alignment_check.rs

@@ -0,0 +1,18 @@
+// Validation makes this fail in the wrong place
+// compile-flags: -Zmiri-disable-validation -Zmiri-seed=0000000000000000
+
+// Even with intptrcast and without validation, we want to be *sure* to catch bugs
+// that arise from pointers being insufficiently aligned. The only way to achieve
+// that is not not let programs exploit integer information for alignment, so here
+// we test that this is indeed the case.
+fn main() {
+    let x = &mut [0u8; 3];
+    let base_addr = x as *mut _ as usize;
+    let u16_ref = unsafe { if base_addr % 2 == 0 {
+        &mut *(base_addr as *mut u16)
+    } else {
+        &mut *((base_addr+1) as *mut u16)
+    } };
+    *u16_ref = 2; //~ ERROR tried to access memory with alignment 1, but alignment 2 is required
+    println!("{:?}", x);
+}