CoroSplit: Replace ad-hoc implementation of reachability with API from CFG.h

The current ad-hoc implementation used to determine whether a basic
block is unreachable doesn't work correctly in the general case (for
example it won't detect successors of unreachable blocks as
unreachable). This patch replaces it with the correct API that uses a
DominatorTree to answer the question correctly and quickly.

rdar://77181156

Differential Revision: https://reviews.llvm.org/D102963