refactor: port simple ground expr extraction from IR to LCNF

[hargoniX]

This PR ports the simple ground expression extraction pass from IR to LCNF.

I locally confirmed that this produces no diff between stage1/stage2 at the C level (apart from the
changed compiler files) so this should essentially be binary equivalent.

[hargoniX]

!bench

[leanprover-radar]

Benchmark results for 3f65b0d against 846420d are in! @hargoniX

• 🟥 build//instructions: +619.7M (+0.00%)

Small changes (1✅)

• ✅ build/module/Lean.Compiler.IR//instructions: -40.8M (-5.44%) (reduced significance based on *//lines)

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 846420dabab15cec6d4c095069dc34d7a9ba4b59 --onto c595413fccaa47ebdc399d9b817c17425687fc26. You can force Mathlib CI using the force-mathlib-ci label. (2026-02-26 15:05:58)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 846420dabab15cec6d4c095069dc34d7a9ba4b59 --onto 8038a8b8904f89ad9542c8eda11379f8f006eab1. You can force reference manual CI using the force-manual-ci label. (2026-02-26 15:06:00)