Step 3 of the mini-compiler: add the generators for the shift and mul routines.

Change-Id: I981d5b7086262c740036f5db768d3e63083984e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/664937


Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>