Add code gen category enum for operator

[eopXD]

Currently under Operator.cpp code gen for different operations are done by a bunch of branching statements, and the branching statements are based on the OP_ID of the operations (e.g. _vv, _vx, or even specialized for a certain operator if (op->opType == "msbf_m")).

This is rather troublesome and bugs may hide inside type-o of these statements.

A better implementation would be have enums that represents categories of these code gen and appending the enun as a parameter inside the definition file of the operator.

Moreover, if the operator already contains codegen category enum, then the function generateOperatorCode can be template-ized.

template<enum CodeGenType>
void generateOperatorCode() { /* ... */ }

This issue can be solved by:

1. build a chart mapping operators to specific code gen pattern
2. adjust generation of definition file
3. rewrite code under Operator.cpp to utilize the appended information