cmd/compile/internal/types: separate printing of structs and tuples

cmd/compile/internal/types: separate printing of structs and tuples

This CL changes the pretty printer to not rely on parameter lists
being represented as TSTRUCTs.

Change-Id: Ie2b8192ee07b96ffbe224e5d98a335368f47abc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/521316
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>