[PeepholeOptimizer] Don't assume bitcast def always has input

Summary:
If we have a MI marked with bitcast bits, but without input operands,
PeepholeOptimizer might crash with assert.

eg:
If we apply the changes in PPCInstrVSX.td as in this patch:

[(set v4i32:$XT, (bitconvert (v16i8 immAllOnesV)))]>;

We will get assert in PeepholeOptimizer.

```
llvm-lit llvm-project/llvm/test/CodeGen/PowerPC/build-vector-tests.ll -v

llvm-project/llvm/include/llvm/CodeGen/MachineInstr.h:417: const
llvm::MachineOperand &llvm::MachineInstr::getOperand(unsigned int)
const: Assertion `i < getNumOperands() && "getOperand() out of range!"'
failed.
```

The fix is to abort if we found out of bound access.

Reviewers: qcolombet, MatzeB, hfinkel, arsenm

Reviewed By: qcolombet

Subscribers: wdng, arsenm, steven.zhang, wuzish, nemanjai, hiraditya, kbarton, MaskRay, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65542

llvm-svn: 369261