[dev.typeparams] cmd/compile/internal/types2: handle untyped constant arithmetic overflow

Factor out the existing "constant representation" check after
untyped constant arithmetic and combine with an overflow check.

Use a better heuristic for determining the error position if we
know the error is for a constant operand that is the result of an
arithmetic expression.

Related cleanups.

With this change, untyped constant arithmetic reports an error
when (integer) constants become too large (> 2048 bits). Before,
such arithmetic was only limited by space and time.

Change-Id: Id3cea66c8ba697ff4c7fd1e848f350d9713e3c75
Reviewed-on: https://go-review.googlesource.com/c/go/+/287832


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