[SPARK-20109][MLLIB] Rewrote toBlockMatrix method on IndexedRowMatrix

## What changes were proposed in this pull request?

- ~~I added the method `toBlockMatrixDense` to the IndexedRowMatrix class. The current implementation of `toBlockMatrix` is insufficient for users with relatively dense IndexedRowMatrix objects, since it assumes sparsity.~~

EDIT: Ended up deciding that there should be just a single `toBlockMatrix` method, which creates a BlockMatrix whose blocks may be dense or sparse depending on the sparsity of the rows. This method will work better on any current use case of `toBlockMatrix` and doesn't go through `CoordinateMatrix` like the old method.

## How was this patch tested?

~~I used the same tests already written for `toBlockMatrix()` to test this method. I also added a new additional unit test for an edge case that was not adequately tested by current test suite.~~

I ran the original `IndexedRowMatrix` tests, plus wrote more to better handle edge cases ignored by original tests.

Author: John Compitello <johnc@broadinstitute.org>

Closes #17459 from johnc1231/johnc-fix-ir-to-block.