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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> @artagnon let me know once there's a stable point for the patches, then I can run them on a larger set of programs to check the wider impact

Will do, thanks! Sorry about the heavy load of notifications from pushes, while this is still in development: I didn't know to check `size-text` in advance.

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