r1cs-compiler: constant-operand branches in AND/XOR black-box handler are unreachable dead code

Description

process_binop_opcode in the R1CS compiler contains match arms for handling ConstantOrACIRWitness::Constant inputs to AND/XOR black-box opcodes. These branches are never reachable in practice because nargo never emits AND/XOR black-box opcodes with constant operands — it either folds them at compile time (both constant) or promotes the constant to a witness (one constant, one witness).

Root Cause

process_binop_opcode has three match arms:

Nargo's behaviour (confirmed via ACIR bytecode inspection):

• Both constants → computed at compile time → emits AssertZero only, no black-box opcode
• One constant + one witness → constant is embedded as a constant-valued witness in the witness table; the ACIR FunctionInput is a Witness reference, not a Constant

So ConstantOrACIRWitness::Constant is never produced by nargo for AND/XOR inputs, and lines L341–L395 and L400–L424 are unreachable.

Affected Files

Read process_binop_opcode in provekit/r1cs-compiler/src/noir_to_r1cs.rs, especially the two ConstantOrACIRWitness::Constant arms identified in the issue. Confirm the AND/XOR inputs produced by nargo through the ACIR bytecode behavior described; the work is done when the unreachable branches are removed without affecting the reachable Witness/Witness handling.