CCP improvements

• Adds asserts that a reachable if/match has a reachable condition/sum
• Handles constant propagation of reduce, which takes into account whether the reduct value is reachable and if not ignores it for constant propagation purposes; this allows CCP to identify constant reduces and eliminate them. Also asserts that if a reduce is reachable its initial value must be as well.

requested review from @rarbore2

assigned to @aaronjc4

approved this merge request

merged

mentioned in commit 0459c0e4