[clang][CFG] Fix 2 memory errors in interval computation.

This fixes 2 bugs and adds corresponding tests. Both related to unreachable
blocks. One occured in the `WTOCompare` construction, which assumed the size of
the order was the same as the number of blocks in the CFG, which isn't true when
some blocks are unreachable.  The other assumed predecessor pointers were
non-null, which can be false for blocks with unreachable predecessors.

Differential Revision: https://reviews.llvm.org/D157033