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 patch converts InstrInterval class to a class template and renames
InstrInterval to Itnerval.
This change will allow us to reuse the Interval for dependency graph
nodes.
