cmd/compile: minor simplifications in rulegen

First, be consistent about declaring typ as &b.Func.Config.Types and
not &config.Types. Not particularly better, and it barely changes the
output, but we're more consistent now.

Second, remove a bit of duplication when handling the typ, auxint, and
aux variables.

Third and last, remove a stray canFail assignment; we ended up setting
that in add, not breakf, so it's not necessary to set it manually if we
don't use breakf.

Updates #33644.

Change-Id: I75999cb223a201969266fbfeae043599fa27fac5
Reviewed-on: https://go-review.googlesource.com/c/go/+/196803


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>