[DSE] Fix for a dangling point bug in DeadStoreElimination.

The patch makes sure that the LastThrowing pointer does not point to any instruction deleted by call to DeleteDeadInstruction.

While iterating through the instructions the pass maintains a pointer to the lastThrowing Instruction. A call to deleteDeadInstruction deletes a dead store and other instructions feeding the original dead instruction which also become dead. The instruction pointed by the lastThrowing pointer could also be deleted by the call to DeleteDeadInstruction and thus it becomes a dangling pointer. Because of this, we see an error in the next iteration.

In the patch, we maintain a list of throwing instructions encountered previously and use the last non deleted throwing instruction from the container.

Patch by Ankit <quic_aankit@quicinc.com>

Reviewers: fhahn, bcahoon, efriedma

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D65326