[SROA] Remove Dead Instructions while creating speculative instructions

[SROA] Remove Dead Instructions while creating speculative instructions

The SROA pass tries to be lazy for removing dead instructions that are collected during iterative run of the pass in the DeadInsts list.  However it does not remove instructions from the dead list while running eraseFromParent() on those instructions.

This causes (rare) null pointer dereferences.  For example, in the speculatePHINodeLoads() instruction, in the following code snippet:

```
   while (!PN.use_empty()) {
     LoadInst *LI = cast<LoadInst>(PN.user_back());
     LI->replaceAllUsesWith(NewPN);
     LI->eraseFromParent();
   }
```

If the Load instruction LI belongs to the DeadInsts list, it should be removed when eraseFromParent() is called.  However, the bug does not show up in most cases, because immediately in the same function, a new LoadInst is created in the following line:

```
LoadInst *Load = PredBuilder.CreateAlignedLoad(
         LoadTy, InVal, Alignment,
         (PN.getName() + ".sroa.speculate.load." + Pred->getName()));
```

This new LoadInst object takes the same memory address of the just deleted LI using eraseFromParent(), therefore the bug does not materialize.  In very rare cases, the addresses differ and therefore, a dangling pointer is created, causing a crash.

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D92431