How to use block-wide logic with multiple segments or smaller problem size?

[sh1ng]

There're actually two questions, but I believe they are related.

• How to block-sort only first N elements? This problem usually occur when processing last block. Currently I'm using dummy value to indicate elements that are supposed to be ignored.
• How can I run block-wide algo on a segmented problem? Let's say I've executed cub::BlockDiscontinuity first and then want to reduce every segment?