cmd/compile/internal/ssagen: fix race added in CL 510539

cmd/compile/internal/ssagen: fix race added in CL 510539

The ssagen pass runs concurrently, so it's not safe to mutate global
variables like this.

Instead, turn it into a constant and add an assertion that the
constant has the correct value.

Fixes #62095.

Change-Id: Ia7f07e33582564892d194153ac3d8759429fc9ac
Reviewed-on: https://go-review.googlesource.com/c/go/+/520598
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>