g8r: prep projected positive-mask cones before gatify

[cdleary]

Motivation

• Gatify was producing suboptimal gate graphs for a common cone shape that masks a data bit with not(sle(x,0)), because there was no preparatory PIR rewrite to expose the simpler sign-bit test pattern.
• Introducing a prep-for-gatify canonicalization improves downstream gatification/QoR by collapsing the cone to a simple sign/data-bit conjunction that tools like ABC can reduce far more compactly.

Description

• Add rewrite_projected_positive_mask_to_sign_test to xlsynth-g8r/src/gatify/prep_for_gatify.rs which recognizes the pattern bit_slice(and(bit_slice(x,0,w), sign_ext(not(sle(x,0)), w)), ...) and rewrites the sle into a sign-bit bit_slice so the mask becomes !sign(x) for the projected data bit.
• Invoke the new rewrite inside prep_for_gatify before dead-node cleanup to preserve SSA/textual ordering and enable safe node nil-ing afterwards.
• The rewrite performs pattern checks (operand shapes, widths, literal rhs checks) and uses ir_utils::replace_node_payload to perform an in-place transformation that keeps node ordering valid.
• Add a focused unit test projected_positive_mask_rewrite_collapses_to_sign_test_and_data_bit that constructs the user-reported cone, asserts the optimized IR uses the sign-bit form (and no live sle), and exhaustively validates equivalence across all bits[13] inputs.

Testing

• Ran the focused unit test with cargo test -p xlsynth-g8r projected_positive_mask_rewrite_collapses_to_sign_test_and_data_bit -- --nocapture, which passed.
• Built fuzz bins with cargo check --manifest-path xlsynth-g8r/fuzz/Cargo.toml --bins, which completed successfully (warnings only).
• Ran repository checks pre-commit run --all-files, which completed and reported rustfmt/hooks passing after formatting.
• Executed full test suite with cargo test --workspace, which completed with all tests passing for the workspace at the time of the change.

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[chatgpt-codex-connector]

Require unique use before mutating sle into sign bit

This rewrite replaces sle_nr in place with a sign-bit bit_slice, but there is no use-count guard that the matched sle/not chain is only used by this cone. If sle(x,0) is also consumed elsewhere, those users now see sign(x) instead of x <= 0, causing incorrect behavior. Add single-use checks (like other rewrites in this file) before mutating shared nodes.

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