[GlobalISel][Tablegen] Fix SameOperandMatcher's isIdentical check

During rule optimization, identical SameOperandMatchers are hoisted into a common group,
however previously only one operand index was considered.
Commutable patterns can introduce SameOperandMatcher checks where the second index is commuted,
resulting in a different check that cannot be hoisted.

Reviewed By: qcolombet

Differential Revision: https://reviews.llvm.org/D111506