[dev.simd] simd: add insert_ordered_map

For minor refactoring in simdgen.

Change-Id: If3f247613743a5e37b400437c78f00fe2726aa50
Reviewed-on: https://go-review.googlesource.com/c/go/+/778861
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>