[dev.regabi] cmd/compile: remove Name.Typegen

Just directly set Type.Vargen when declaring defined types within a
function.

Change-Id: Idcc0007084a660ce1c39da4a3697e158a1c615b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/283212


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>