cmd/compile/internal/ir: add Func.DeclareParams

cmd/compile/internal/ir: add Func.DeclareParams

There's several copies of this function. We only need one.

While here, normalize so that we always declare parameters, and always
use the names ~pNN for params and ~rNN for results.

Change-Id: I49e90d3fd1820f3c07936227ed5cfefd75d49a1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/528415
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>