Another CCP bugfix

Fixes analysis of read nodes in CCP; we were previously ignoring the indices but we need to account for their reachability to ensure we properly mark read nodes as unreachable if any of their position arguments are unreachable.

requested review from @rarbore2

assigned to @aaronjc4

mentioned in commit 5d2ed362

merged

Good catch. I think there are a few other nodes w/ data inputs that we previously ignored that may need the same change @aaronjc4 (Write for example?)?

Weirdly, write was accounting for all its data inputs already. Looking at other nodes, reduce, if, and match all ignore their data inputs but I think if their data inputs are unreachable I think that would mean their control inputs must be as well, so it should be okay, but we maybe need to think about that more to be sure.

Might be worth asserting that.