cmd/internal/gc: Toughen escape analysis against some bugs.

Ensures that parameter flow bits are not set for tags EscScope, EscHeap, EscNever;
crash the compiler earl to expose faulty logic, rather than flake out silently downstream.

Change-Id: I1428129980ae047d02975f033d56cbbd04f49579
Reviewed-on: https://go-review.googlesource.com/9601


Reviewed-by: Russ Cox <rsc@golang.org>