[dev.typeparams] cmd/compile/internal/typecheck: declutter generated builtin.go (cleanup)

Even though builtin.go is generated, there's no need for
it to be so huge in terms code size. Nor does ultimate
speed matter here.

Added two simple helper functions that are not inlined,
which reduce the amount of code generated for this file
from 77881 bytes to 27641 bytes of assembly (per compiler
-S output) and reduce the compile binary by ~140KiB
(of course that's insignificant given the 22MiB file size).

Change-Id: I3058ec62788b33eaeff2f9d5fe975b8e41cbf172
Reviewed-on: https://go-review.googlesource.com/c/go/+/287772


Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>