perf: coalescing of RC ops within one basic block

[hargoniX]

This PR introduces coalescing of RC operations to the RC optimizer. Whenever we perform multiple incs for a single value within one basic block it is legal to instead perform all of these incs at once at the first inc side. This is the case because the value will stay alive until at least the last inc and was thus never observable with RC=1. Hence, this change of inc location never destroys reuse opportunities.

[hargoniX]

!bench

[leanprover-radar]

Benchmark results for 5cd1000 against a8bbc95 are in. Significant changes detected! @hargoniX

• ✅ build//instructions: -941.6M (-0.01%)

Large changes (2✅)

• ✅ compiled/deriv//instructions: -72.6M (-0.95%)
• ✅ size/libleanshared.so//bytes: -1MiB (-0.97%)

Medium changes (4✅)

• ✅ compiled/rbmap//instructions: -8.0M (-0.08%)
• ✅ size/Init/.olean//bytes: -391kiB (-0.43%)
• ✅ size/all/.ir//bytes: -2MiB (-0.50%)
• ✅ size/compile/.out//bytes: -9MiB (-0.53%)

Small changes (6✅, 1🟥)

• 🟥 build/profile/compilation (LCNF impure)//wall-clock: +1s (+4.47%)
• ✅ compiled/rbmap_checkpoint//instructions: -8.2M (-0.06%)
• ✅ compiled/rbmap_checkpoint2//instructions: -7.0M (-0.07%)
• ✅ compiled/rbmap_fbip//instructions: -7.5M (-0.10%)
• ✅ size/all/.c//lines: -39.6k (-0.36%)
• ✅ size/all/.olean//bytes: -838kiB (-0.26%)
• ✅ size/install//bytes: -6MiB (-0.23%)

[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 a8bbc95d9fe665255a5c98fb402546233e760636 --onto e6df474dd9c3ad0e21771eaa808c53f66222216d. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-26 14:57:00)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 439c3c55448290dd0b0b4b205afde4229a7cee4d --onto 4786e082dca22873d14d2a5b9b7c8843380c6e78. You can force Mathlib CI using the force-mathlib-ci label. (2026-03-27 10:50:51)

[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 a8bbc95d9fe665255a5c98fb402546233e760636 --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-26 14:57:03)
• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 439c3c55448290dd0b0b4b205afde4229a7cee4d --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-03-27 10:50:52)