cmd/compile: get rid of ugly {Recvs,Params,Results}P methods

cmd/compile: get rid of ugly {Recvs,Params,Results}P methods

These were a hack abstraction for before FuncType existed.

The result value from calling FuncType() could be saved, but this
maintains the current idiom of consistently using t.FuncType().foo
everywhere in case we choose to evolve the API further.

Change-Id: I81f19aaeab6fb7caa2d4da8bf0bbbc358ab970d0
Reviewed-on: https://go-review.googlesource.com/28150
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>