[TableGen][GlobalISel] Fix tblgen optimization bug

When optimizing the table, PointerToAnyOperandMatchers would be
incorrectly reported as identical even though they have different
SizeInBits values. This bug was due to failing to overload the
isIdentical() method, which this patch addresses.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D86199