[PowerPC] Fix the splat immediate in PPCMIPeephole depending on if we have an...

[PowerPC] Fix the splat immediate in PPCMIPeephole depending on if we have an Altivec and VSX splat instruction.

An assertion of the following can occur because Altivec and VSX splats use a different operand number for the immediate:
```
int64_t llvm::MachineOperand::getImm() const: Assertion `isImm() && "Wrong MachineOperand accessor"' failed.
```
This patch updates PPCMIPeephole.cpp assign the correct splat immediate.

Differential Revision: https://reviews.llvm.org/D105790