[dev.regabi] go/types: use 512 bits as max. integer precision

This is a port of CL 288633 to go/types. It differs from that CL
in the implementation of opName, which now uses ast Exprs.

Additionally, a couple tests had to be updated:
 + TestEvalArith is updated to not overflow.
 + stmt0.src is updated to have an error positioned on the '<<'
   operator.

Change-Id: I628357c33a1e7b0bb5bb7de5736f1fb10ce404e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/290630


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