mlir/VectorToGPU: use std::optional (NFC)

This is part of an effort to migrate from llvm::Optional to std::optional:

See also: https://discourse.llvm.org/t/deprecating-llvm-optional-x-hasvalue-getvalue-getvalueor/63716



Signed-off-by: Ramkumar Ramachandra <r@artagnon.com>