Use SetLenOnDrop in Vec::truncate()

This avoids a redundant length check in some cases when calling
`Vec::truncate` or `Vec::clear`.

Fixes #51802

src/test/codegen/vec-clear.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @vec_clear
+#[no_mangle]
+pub fn vec_clear(x: &mut Vec<u32>) {
+    // CHECK-NOT: load
+    // CHECK-NOT: icmp
+    x.clear()
+}