cmd/compile/internal/typecheck: add selector helpers

cmd/compile/internal/typecheck: add selector helpers

This CL refactors common patterns for constructing field and method
selector expressions. Notably, XDotField and XDotMethod are now the
only two functions where a SelecterExpr with OXDOT is constructed.

Change-Id: I4c087225d8b295c4a6a92281ffcbcabafe2dc94d
Reviewed-on: https://go-review.googlesource.com/c/go/+/520979
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>