[dev.simd] simd: refactor rule generation template

Break up large template into map + per-rule templates.
Use ordered map to also encode pattern-specificity
order in a single place.

Change-Id: I349dbcf968b8c869fced9fe1697ead81c14520a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/778862
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>