[dev.typeparams] cmd/compile/internal/types2: use 512 bits as max. integer precision

This matches the compiler's existing limitations and thus ensures
that types2 reports the same errors for oversize integer constants.

Change-Id: I4fb7c83f3af69098d96f7b6c53dbe3eaf6ea9ee4
Reviewed-on: https://go-review.googlesource.com/c/go/+/288633


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