cmd/compile: reduce the regexp work in rulegen

As correctly pointed out by Giovanni Bajo, doing a single regexp pass
should be much faster than doing hundreds per architecture. We can then
use a map to keep track of what ops are handled in each file. And the
amount of saved work is evident:

	name     old time/op         new time/op         delta
	Rulegen          2.48s ± 1%          2.02s ± 1%  -18.44%  (p=0.008 n=5+5)

	name     old user-time/op    new user-time/op    delta
	Rulegen          10.9s ± 1%           8.9s ± 0%  -18.27%  (p=0.008 n=5+5)

	name     old sys-time/op     new sys-time/op     delta
	Rulegen          209ms ±28%          236ms ±18%     ~     (p=0.310 n=5+5)

	name     old peak-RSS-bytes  new peak-RSS-bytes  delta
	Rulegen          178MB ± 3%          176MB ± 3%     ~     (p=0.548 n=5+5)

The speed-up is so large that we don't need to parallelize it anymore;
the numbers above are with the removed goroutines. Adding them back in
doesn't improve performance noticeably at all:

	name     old time/op         new time/op         delta
	Rulegen          2.02s ± 1%          2.01s ± 1%   ~     (p=0.421 n=5+5)

	name     old user-time/op    new user-time/op    delta
	Rulegen          8.90s ± 0%          8.96s ± 1%   ~     (p=0.095 n=5+5)

While at it, remove an unused method.

Change-Id: I328b56e63b64a9ab48147e67e7d5a385c795ec54
Reviewed-on: https://go-review.googlesource.com/c/go/+/195739


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