[mlir][Linalg] Add loop.parallel lowering for all Linalg Ops.

The outer parallel loops of a linalg operation is lowered to
loop.parallel, with the other loops lowered to loop.for. This gets the
lowering to loop.parallel on par with the loop.for lowering. In future
the reduction loop could also be lowered to loop.parallel.
Also add a utility function that returns the loops that are
created.

Differential Revision: https://reviews.llvm.org/D77678