[PowerPC] PPCBoolRetToInt: Don't translate Constant's operands

When collecting `i1` values via `findAllDefs`, ignore Constant's
operands, since Constant's operands might not be `i1`.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46923 which causes ICE
```
llvm-project/llvm/lib/IR/Constants.cpp:1924: static llvm::Constant *llvm::ConstantExpr::getZExt(llvm::Constant *, llvm::Type *, bool): Assertion `C->getType()->getScalarSizeInBits() < Ty->getScalarSizeInBits()&& "SrcTy must be smaller than DestTy for ZExt!"' failed.
```

Differential Revision: https://reviews.llvm.org/D85007