Fix MLIR gcc7 build: ambiguous overload from user conversion

The implicit conversion from an op to a Value is not triggered for some reasons:

/LoopLikeSCFOpsTest.cpp:78:57: error: call of overloaded ‘ValueRange(mlir::arith::ConstantIndexOp)’ is ambiguous
       b.create<scf::ParallelOp>(loc, ValueRange(lb.get()), ValueRange(ub.get()),

explicitly taking the result of the op should solve it.