[wraitii]

Stop spill-range trimming when it would enter a deeper loop than the split point, on either side of the split. This prevents preservation around a call outside a loop from introducing unnecessary stores and reloads inside that loop.

See #271 for details. This PR was written by GPT-6 Astra, the fix looks reasonable to me but I can't say I have a deep understanding of this codebase. It's also a heuristic, as explained in the issue.

Validation that Astra ran:

• All regalloc2 tests pass, plus 6,799 randomized allocation checks.
• Both versions pass 1,399 Cranelift filetests and 1,196 checker-enabled files; existing assembly snapshots are unchanged.
• Five targeted execution checks pass on both versions.
• Sightglass: all 29 PCA workloads, 20 samples per engine on Apple ARM64. No statistically significant compilation or execution regression detected.