[llvm] [CodeGen] Reland "Use SmallMapVector for SpillPlacement::Node::Links" (PR #196270)

[Nikita Popov via llvm-commits]

nikic wrote:

Looks like increasing the small size to 8 reduces the overhead a bit: https://llvm-compile-time-tracker.com/compare.php?from=a342f64eb88c604be615cbdd3d2fdfff70f12d7e&to=295b39a07d9216d9bc682a89303ace98a01a4a3d&stat=instructions:u

I'll check 16 as well.

https://github.com/llvm/llvm-project/pull/196270