[mlir][python] fix constructor generation for optional operands in presence of segment attribute

The ODS-based Python op bindings generator has been generating incorrect
specification of the operand segment in presence if both optional and variadic
operand groups: optional groups were treated as variadic whereas they require
separate treatement. Make sure it is the case. Also harden the tests around
generated op constructors as they could hitherto accept the code for both
optional and variadic arguments.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D113259