V4 IR: clean up DefUses(Calculator)

Previously, the DefUsesCalculator was a pass over the IR to find all
definitions and uses of temps. This is now merged with the variable
renaming when transforming into SSA form, where all definitions and all
uses are already visited (and changed). As it no longer calculates
anything, the class is also renamed to DefUses.

The interface is also cleaned up, as are all usages. This involved some
small changes to the data-structures storing return values from DefUses.

Change-Id: I40e665f5dee6144fc81066fbf6950355ebe8dfa4
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>