[llvm] [MC] Replace fragment ilist with singly-linked lists (PR #95077)

[Fangrui Song via llvm-commits]

MaskRay wrote:

https://llvm-compile-time-tracker.com/compare.php?from=48f8130a49aad715ff6d5136dad2447d41e9537b&to=075c26f8b177e976f44c8c4dd6a95e80ee7a9510&stat=max-rss&linkStats=on

|Benchmark|Old|New|
|--- |--- |--- |
|kimwitu++|234MiB|233MiB (-0.78%)|
|sqlite3|221MiB|219MiB (-0.82%)|
|consumer-typeset|158MiB|157MiB (-0.22%)|
|Bullet|197MiB|196MiB (-0.20%)|
|tramp3d-v4|634MiB|627MiB (-0.99%)|
|mafft|119MiB|119MiB (+0.08%)|
|ClamAV|231MiB|229MiB (-0.74%)|
|lencod|272MiB|271MiB (-0.64%)|
|SPASS|293MiB|293MiB (-0.20%)|
|7zip|458MiB|454MiB (-0.88%)|
|geomean|252MiB|250MiB (-0.54%)|


https://github.com/llvm/llvm-project/pull/95077