[dev.regabi] cmd/compile: always use a Field for ODOTPTR expressions

During walk, we create ODOTPTR expressions to access runtime struct
fields. But rather than using an actual Field for the selection, we
were just directly setting the ODOTPTR's Offset field.

This CL changes walk to create proper struct fields (albeit without
the rest of their enclosing struct type) and use them for creating the
ODOTPTR expressions.

Passes toolstash -cmp.

Change-Id: I08dbac3ed29141587feb0905d15adbcbcc4ca49e
Reviewed-on: https://go-review.googlesource.com/c/go/+/280432


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>