[dev.regabi] cmd/compile: simplify typecheckdef

Reorganize code to be a little clearer. Also allows tightening
typecheckdefstack from []ir.Node to []*ir.Name.

Passes toolstash -cmp.

Change-Id: I43df1a5e2a72dd3423b132d3afe363bf76700269
Reviewed-on: https://go-review.googlesource.com/c/go/+/280649


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>