[Synth] Add canonicalization for synth.choice

[joaovam]

Resolves #9932

This PR implements the transitive merge canonicalization for the synth.choice operation.

How it works:
It uses a worklist algorithm to compute the transitive closure of connected synth.choice operations. By traversing both the definitions and the users, it flattens nested or shared choices into a single, unified operation while automatically deduplicating redundant operands.

Example

// Before:
%0 = synth.choice %x, %y, %z
%1 = synth.choice %0, %u
%2 = synth.choice %z, %v

// After:
%merged_choice = synth.choice %x, %y, %z, %u, %v