cmd/compile/internal/types2: rename Checker.cycle to Checker.validCycle
Also, invert the boolean result. This matches Checker.validType;
it's also easier to understand. Preparation for the next CL which
detects cycles through type parameter lists.
Change-Id: I00a75d2359ca20827c9bf406945508716c826fc4
Reviewed-on: https://go-review.googlesource.com/c/go/+/362336
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>