[MLIR] Don't insert YieldOp for non-void loop.for by default.

The ForOp::build ensures that there is a block terminator which is great for
the default use case when there are no iter_args and loop.for returns no
results.  In non-zero results case we always need to call replaceOpWithNewOp
which is not the nicest thing in the world. We can stop inserting YieldOp when
iter_args is non-empty. IfOp::build already behaves similarly.