cmd/compile/internal/types: add example test for type inference

Follow-up on https://golang.org/cl/308372.

For #44799.

Change-Id: I27d149241d465ee31e9c21d5f0935f31efcb5b60
Reviewed-on: https://go-review.googlesource.com/c/go/+/308973


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