[DDR] Introduce implicit equality check for the source pattern operands with the same name.

This CL allows user to specify the same name for the operands in the source pattern which implicitly enforces equality on operands with the same name.
E.g., Pat<(OpA $a, $b, $a) ... > would create a matching rule for checking equality for the first and the last operands. Equality of the operands is enforced at any depth, e.g., OpA ($a, $b, OpB($a, $c, OpC ($a))).

Example usage: Pat<(Reshape $arg0, (Shape $arg0)), (replaceWithValue $arg0)>

Note, this feature only covers operands but not attributes.
Current use cases are based on the operand equality and explicitly add the constraint into the pattern. Attribute equality will be worked out on the different CL.

Differential Revision: https://reviews.llvm.org/D89254