cmd/compile: fix generic.rules

generic.rules wasn't updated when rewritegeneric.go was. This commit
updates it so that the rewritegeneric.go file can be regenerated.

Fixes #18885.

Change-Id: Ie7dab653ca0a9ea1c255fd12e311a0d9e66afdd2
Reviewed-on: https://go-review.googlesource.com/36032


Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>