Apply static bounds checking logic during type checking even to
zero-element arrays, but skip synthesized OINDEX nodes that the
compiler has asserted are within bounds (such as the ones generated
while desugaring ORANGE nodes).  This matches the logic in walkexpr
that also skips static bounds checking when Bounded is true.

Passes toolstash/buildall.

Fixes #12944.

Change-Id: I14ba03d71c002bf969d69783bec8d1a8e10e7d75
Reviewed-on: https://go-review.googlesource.com/15902Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>