The DAG will now receive a callback whenever a new instruction is
created and will update itself accordingly.

This patch implements the notifier for Instruction intervals. It updates
the interval's top/bottom.

This patch implements `Interval::comesBefore(const Interval &Other)`
which returns true if this interval is strictly before Other in program
order. The function asserts that the intervals are disjoint.

This patch implements a few set operations for the intervals. These
include:
- operator==() and operator!=() for comparing two intervals.
- disjoint()
- intersection()
- difference, which uses operator-()

This reverts commit 8dfeb4ef5d60a5c764f0ce249cc4ec69e012ff93.

This reverts commit ca47f48a5c9e81ef8b5c4a5b1fbc473ea5d5497c.