add codegen test

src/test/codegen/vec-in-place.rs

@@ -0,0 +1,13 @@
+// compile-flags: -O
+// min-llvm-version: 11.0
+#![crate_type = "lib"]
+
+// Ensure that trivial casts of vec elements are O(1)
+
+// CHECK-LABEL: @vec_iterator_cast
+#[no_mangle]
+pub fn vec_iterator_cast(vec: Vec<isize>) -> Vec<usize> {
+    // CHECK-NOT: loop
+    // CHECK-NOT: call
+    vec.into_iter().map(|e| e as usize).collect()
+}