cmd/compile: normalize whitespace around square brackets
cmd/compile: normalize whitespace around square brackets

I noticed some instances of "[ " and " ]" in the rewrite rules.
Normalizing them helps catch possible future duplicate rules.

Change-Id: I892fd7e9b4019ed304f0a61fa2bb7f7e47ef8f38
Reviewed-on: https://go-review.googlesource.com/c/go/+/213682
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,nawawi/go,akutz/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,akutz/go,akutz/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,golang/go,golang/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,akutz/go,golang/go,akutz/go,nawawi/go,cloudflare/go