Fix transmute intrinsic mir validation ICE

tests/ui/mir/validate/transmute_cast_sized.rs

@@ -0,0 +1,17 @@
+// build-pass
+// compile-flags: -Zvalidate-mir
+// edition: 2021
+
+#![crate_type = "lib"]
+
+// Use `PhantomData` to get target-independent size
+async fn get(_r: std::marker::PhantomData<&i32>) {
+    loop {}
+}
+
+pub fn check() {
+    let mut v = get(loop {});
+    let _ = || unsafe {
+        v = std::mem::transmute([0_u8; 1]);
+    };
+}