[ConstraintElimination] Remove dead variables when dropping constraints.

This patch extends ConstraintElimination to also remove dead variables
when removing a constraint. When a constraint is removed because it is
out of scope, all new variables added for this constraint can also be
removed.

This keeps the total size of the systems much smaller, because it
reduces the number of variables drastically.

It also fixes a bug where variables where removed incorrectly.

Fixes https://github.com/llvm/llvm-project/issues/54228