cmd/compile: disable rewrite loop detector for deadcode-only changes

We're guaranteed we won't infinite loop on deadcode-only changes,
because each change converts valid -> invalid, and there are only a
finite number of valid values.

The loops this test is looking for are those generated by rule
applications, so it isn't useful to check for loops when rules aren't
involved.

Fixes #51639

Change-Id: Idf1abeab9d47baafddc3a1197d5064faaf07ef78
Reviewed-on: https://go-review.googlesource.com/c/go/+/392760


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Trust: Josh Bleecher Snyder <josharian@gmail.com>