[llvm] ConstraintSystem: replace data structure with Matrix (PR #98895)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

It looks like, due to the small Matrix size, the gains were all observed in the dry-run cutoff. Subtracting the impact of the dry-run patch, switching the data structure to Matrix has no impact according to LLVM Compile Time Tracker: http://llvm-compile-time-tracker.com/compare.php?from=47c8510fd2749c1f44cc33db75f7eac142919e71&to=15dd68506d479cc50349de1cf1e70bd896b9c82e&stat=instructions:u.

https://github.com/llvm/llvm-project/pull/98895