Followup: Potential indexing performance improvement

This is a followup to #1030 which made modifications to the way we indexed into a DefaultRectangular array. Performance results indicated that while #1030 positively affected some tests, others regressed.

This patch adds a config param flag that lets users assert that there is no slicing in their program. If there is no slicing, we can avoid using the conditional from #1030, which may lead to better performance. Otherwise, we use the original method of multiplying ind by blk. This flag is off by default.