[clang][dataflow] Fix a null pointer crash in `computeBlockInputState()`.

The crash was due to unconditionally calling
`Block.succ_begin()->getReachableBlock()->hasNoReturnElement()`, but
`getReachableBlock()` can return null now that we have turned
`PruneTriviallyFalseEdges` on.

This patch adds two tests that crash without the fix.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D151071