cmd/compile: stenciled conversions might be NOPs

A generic conversion might be required for when converting T->interface{}.
When stenciled with T=interface{}, then that conversion doesn't need
to do anything.

Fixes #48276

Change-Id: Ife65d01c99fbd0895cb7eec79df9e93e752b1fa5
Reviewed-on: https://go-review.googlesource.com/c/go/+/348736


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>