cmd/compile: simplify various signature type handling code

cmd/compile: simplify various signature type handling code

Make more use of RecvParams and RecvParamsResults helper methods.

Also, correct misuse of Go spec terminology ("return" is a statement;
"result" is the class of parameters that appear in a function type).

Change-Id: I94807a747c494c9daa5441da7d9e3aea77aae33b
Reviewed-on: https://go-review.googlesource.com/c/go/+/521395
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>