cmd/compile: remove badgerbadgerbadger optimization

As discussed in #29242, this optimization is for a bash-ism.
No one writes Go code like this.

In this repo, it triggers only in test/fixedbugs/bug425.go
and that appears to be accidental.

Fixes #29242

Change-Id: I257e6ecc73f24680f7282c6ab28729de4e8b27af
Reviewed-on: https://go-review.googlesource.com/c/163728


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>