Verify dominance relations in IR graph

This MR implements the following:

• Verify dominance properties for IR (fork / join dominance / postdominance, phis/thread_ids/collects dominate uses, thread_ids uses are postdominated by corresponding join)
• Change control type to explicitly storing fork nodes, rather than fork factors

requested review from @aaronjc4

assigned to @rarbore2

approved this merge request

added 1 commit

• f359260a - fix

Compare with previous version

resolved all threads

mentioned in commit 4d453ec5

merged