add a codegen check for absence of memcpy in String::push

tests/codegen/string-push.rs

@@ -0,0 +1,11 @@
+//! Check that `String::push` is optimized enough not to call `memcpy`.
+
+//@ compile-flags: -O
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @string_push_does_not_call_memcpy
+#[no_mangle]
+pub fn string_push_does_not_call_memcpy(s: &mut String, ch: char) {
+    // CHECK-NOT: call void @llvm.memcpy
+    s.push(ch);
+}