Add extended basis rotation passes for OpenFHE (ConvertToExtendedBasis + HoistKeySwitchDown)

[toprakmurat]

Summary

Continuation of #2574, related to #2519.
Adds two new passes for optimizing rotation-heavy FHE programs by deferring expensive key-switch operations:

1. --openfhe-convert-to-extended-basis - Converts fast_rotation → fast_rotation_ext + key_switch_down
2. --openfhe-hoist-key-switch-down

Changes

Pass 1: ConvertToExtendedBasis

• Converts each openfhe.fast_rotation to openfhe.fast_rotation_ext + openfhe.key_switch_down
• Semantically equivalent transformation that enables further optimization

Pass 2: HoistKeySwitchDown

• Pushes key_switch_down operations later in the IR to reduce key-switch count.
• Works on Add, AddInPlace, and Mul operations.
• When at least one operand is key_switch_down (with single use), the pass swaps:

// Case 1: Both operands are key_switch_down
%a = key_switch_down(%a_ext)
%b = key_switch_down(%b_ext)
%sum = add(%a, %b)

to:

%sum_ext = add(%a_ext, %b_ext)
%sum = key_switch_down(%sum_ext)

and,

// Case 2: One operand is key_switch_down (enables chained optimization)
%a = key_switch_down(%a_ext)
%sum = add(%a, %ct)  // %ct is not key_switch_down

to:

%sum_ext = add(%a_ext, %ct)
%sum = key_switch_down(%sum_ext)

Through greedy iteration, this reduces multiple key-switches in chained operations to a single final key-switch.

[toprakmurat]

Hi @asraa, I'd appreciate your review of these changes when you have time. Should I also add interpreter benchmarks comparing performance with and without these passes?

[j2kun]

@AlexanderViand Asra and I are a bit strapped for time these days. Do you think you'd have time to review this PR?

[j2kun]

LGTM! Some minor changes suggested. In my view the next step would be to add these two passes to the openfhe pipeline, and run them with something like a matvec op that lowers to an IR with lots of rotations and summations, which should be able to take advantage of these patterns.

[j2kun]

Also when you're ready, please squash your commits into one, as per https://heir.dev/docs/development/#pull-request-review-flow

[j2kun]

Thank you @toprakmurat!