Be more consistent about this.  There's no reason to do the
pointer arithmetic on a different type, as sizeof(int) >=
sizeof(ptr) on all of our platforms.  It simplifies our
rewrite rules also, except for a few that need duplication.

Add some more constant folding to get constant indexing and
slicing to fold down to nothing.

Change-Id: I3e56cdb14b3dc1a6a0514f0333e883f92c19e3c7
Reviewed-on: https://go-review.googlesource.com/16586
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>