[dev.typeparams] cmd/compile/internal/types2: fix a panic in missingMethod

When static == false, missingMethod incorrectly continues with a nil
Func.

Also remove some unnecessary type names from typeterm_test.go, which was
done in the go/types port.

Change-Id: I21fa637ac82b115563d3601314a470a5a43f9ae0
Reviewed-on: https://go-review.googlesource.com/c/go/+/339672


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>