Added an optional unitAttr "commutative" to the pdl.operation op. If
present, it specifies to the matcher that the operation should be
treated as commutative w.r.t. all its operands.
Co-authors: Srishti Srivastava, Prateek Gupta
Signed-off-by: Srishti Srivastava <srishti.srivastava@polymagelabs.com>