Rethink how to perform merge for GatewayIR on provider specifics.

[sawsa307]

What would you like to be added:
Find a better way to merge GatewayIR. Currently we delegate the merge logic to each provider since the merge only involves provider-specific logic, but there may be an alternative way to make more clean and straightforward.

Why this is needed:
To ensure GatewayIR are merged in a easy-to-read and maintainable manner.

[sawsa307]

/cc @LiorLieberman

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