[coroutines] PR33271: Remove stray coro.save intrinsics during CoroSplit

Summary:
Optimization passes may remove llvm.coro.suspend intrinsic while leaving matching llvm.coro.save intrinsic orphaned.
Make sure we clean up orphaned coro.saves.  The bug manifested with a crash similar to this:

```
    llvm_unreachable("Unknown type!");
    llvm::MVT::getVT (Ty=0x489518, HandleUnknown=false)
    llvm::EVT::getEVT
    llvm::TargetLoweringBase::getValueType
    llvm::ComputeValueVTs
    llvm::SelectionDAGBuilder::visitTargetIntrinsic
```

Reviewers: GorNishanov

Subscribers: EricWF, llvm-commits

Differential Revision: https://reviews.llvm.org/D33817

llvm-svn: 304518