[dev.regabi] cmd/compile: remove nested functions from expands_calls.go

Replace nested function spaghetti with state object and methods.
Still somewhat complex, but a bit more explicit.

Change-Id: I21987c8e4be75821faa5a248af05d2095cdfb0d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/287132


Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>