Fix bug for Ops with default valued attributes and successors/variadic regions.

When both a DefaultValuedAttr and a successor or variadic region was specified, this would generate invalid C++ declaration. There would be the parameter with a default value, followed by the successors/regions, which don't have a default, which is invalid.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D110205