[FIRRTL] Add folders for IntegerAddOp

[mikeurbach]

This is a primitive Property operation, defined by FIRRTL spec section 25.1.1. General folds for signed arbitrary precision integer arithmetic can be applied, but none were defined in the initial PR. This could include things like:

• integer_add(a, 0) -> a
• integer_add(constantA, constantB) -> constantA + constantB
• etc...

[sazimi13]

Hey I would like to take this issue up, after completing can hopefully take the other 2 folding issues as well.

[sazimi13]

Can you please provide the FIRRTL spec that you are referring to? @mikeurbach

[mikeurbach]

Yep, the FIRRTL spec is located here: https://github.com/chipsalliance/firrtl-spec/blob/main/spec.md#integer-add-operation. That link points to the section for the integer add operation.