[dev.typeparams] cmd/compile: refactor SelectorExpr code into helpers
This CL refactors the SelectorExpr-handling code added in CL 285373
into helper functions that can eventually be reused by iimport.
Change-Id: I15b4a96c242f63cb370d7492ed08168550724f47
Reviewed-on: https://go-review.googlesource.com/c/go/+/285953
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>