Avoid reloading Vec::len across grow_one in push

This saves an extra load from memory.

tests/codegen/vec-len-invariant.rs

@@ -0,0 +1,16 @@
+//@ compile-flags: -O
+//@ only-64bit
+//
+// This test confirms that we do not reload the length of a Vec after growing it in push.
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @should_load_once
+#[no_mangle]
+pub fn should_load_once(v: &mut Vec<u8>) {
+    // CHECK: load i64
+    // CHECK: call {{.*}}grow_one
+    // CHECK-NOT: load i64
+    // CHECK: add {{.*}}, 1
+    v.push(1);
+}