cmd/compile: avoid ifaceeq call if we know the interface is direct

cmd/compile: avoid ifaceeq call if we know the interface is direct

We can just use == if the interface is direct.

Fixes #70738

Change-Id: Ia9a644791a370fec969c04c42d28a9b58f16911f
Reviewed-on: https://go-review.googlesource.com/c/go/+/635435
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>