cmd/compile: add variable length TRESULTS type for SSA use.

This type is very much like TTUPLE, but not just for pairs.
Used to describe results of a pre-expansion function call.
(will later probably also be used to describe the incoming args).

Change-Id: I811850cfcc2b3de85085eb4c2eca217c04c330b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/242360


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