Missing rule to fold out Convert ops on 32-bit architectures.

This comes up with expressions like
unsafe.Pointer(uintptr(p) + x)

Change-Id: I429e968e5d1a3e13a386ddd29a08ebb6d7dd938a
Reviewed-on: https://go-review.googlesource.com/112158Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: David Chase <drchase@google.com>