[dev.regabi] cmd/compile: simplify some tree traversal code

When looking for referenced functions within bottomUpVisitor and
initDeps, the logic for ODOTMETH, OCALLPART, and OMETHEXPR are
basically identical, especially after previous refactorings to make
them use MethodExprName. This CL makes them exactly identical.

Passes toolstash -cmp.

Change-Id: I1f59c9be99aa9484d0397a0a6fb8ddd894a31c68
Reviewed-on: https://go-review.googlesource.com/c/go/+/280441


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