MC: Eliminate redundant fragment relaxation
The relaxOnce function now returns the index of the last modified section,
allowing subsequent calls to skip already stable sections.
This optimization can often save redundant iteration for trailing
.debug_ sections, leading to minor instructions:u decrease.
https://llvm-compile-time-tracker.com/compare.php?from=aec88832df5e8c1dcbe259a6cb3d82d44b89ff23&to=8012fb16eff93cba48e5f08166762c5333bd1d42&stat=instructions:u
