Eliminate branches: implement isl analysis and recurse into conditionals

TODO

• Implement some tests
• Shall we make the isl analysis optional? (Previously it was so probably yes.)

In combination with loop cutting/peeling I noticed a difference to the old backend. Previously loop counters got inlined, so the AST contained expressions like

if (n < n) {
  // ...
}

which got elided by ISL.

Now, the loop counter is declared as a variable, i.e.

int i = n;
if (i < n) {
  // ...
}

which can not be removed because we do not analyze declarations/assignments.

I think this is OK because these conditionals do not appear inside the loop. Anyway, I wanted to mention it.

src/pystencils/backend/transformations/eliminate_branches.py

@@ -109,7 +109,7 @@ class EliminateBranches:
         self, conditional: PsConditional, ec: BranchElimContext
     ) -> PsConditional | PsBlock | None:
         condition_simplified = self._elim_constants(conditional.condition)
-        if not self.no_isl:
+        if not self._no_isl:
             condition_simplified = self._isl_symplify_condition(
                 condition_simplified, ec
             )
@@ -125,7 +125,10 @@ class EliminateBranches:
     def _isl_symplify_condition(
         self, condition: PsExpression, ec: BranchElimContext
     ) -> PsExpression:
-        """Transformations using integer sets based on ISL library"""
+        """If installed, use ISL to simplify the passed condition to true or
+        false based on enclosing loops and conditionals. If no simplification
+        can be made or ISL is not installed, the original condition is returned.
+        """
 
         try:
             import islpy as isl