Handle UnreachableInst in isGuaranteedToTransferExecutionToSuccessor

A block with an UnreachableInst does not transfer execution to a successor.
The problem was exposed by GVN-hoist. This patch fixes bug 32153.

Patch by Aditya Kumar.

Differential Revision: https://reviews.llvm.org/D30667

llvm-svn: 297254