cmd/compile: make no-op rewrite funcs smaller

cmd/compile: make no-op rewrite funcs smaller

This doesn't change any behavior, but should help the compiler realise
that these funcs really do nothing at all.

Change-Id: Ib26c02ef264691acac983538ec300f91d6ff98db
Reviewed-on: https://go-review.googlesource.com/c/go/+/280314
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>