cmd/compile: use slices.{Sort,SortFunc}

cmd/compile: use slices.{Sort,SortFunc}

Now that we're bootstrapping from a toolchain that has the slices
package.

Updates #64751

Change-Id: I2e63d95577d058670d3dc75bd45d6e050c6f0e25
Reviewed-on: https://go-review.googlesource.com/c/go/+/610601
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>