cmd/compile/internal/types2: minor cleanups in predicates.go

- reordered some functions for better organization
- renamed single arguments typ to t for consistency
- updated some comments

No functional changes.

Change-Id: I4362ac48044595cdf5c3d9eb7b2f7b94e776d65b
Reviewed-on: https://go-review.googlesource.com/c/go/+/360956


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>