cmd/compile: add sliceBound

cmd/compile: add sliceBound

Add a constant for the magic -1 for slice bounds.
Use it.
Enforce more aggressively that bounds must be
slice, ddd, or non-negative.
Remove ad hoc check in plive.go.
Check bounds before constructing an array type
when typechecking.

All changes are manual.

Passes toolstash -cmp.

Change-Id: I9fd9cc789d7d4b4eea3b30b24037a254d3788add
Reviewed-on: https://go-review.googlesource.com/21348
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>