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

@@ -67,9 +67,9 @@ class EliminateBranches:
                 statements_new: list[PsAstNode] = []
                 for stmt in statements:
                     if isinstance(stmt, PsConditional):
-                        result = self.visit(stmt, ec)
-                        if not result.structurally_equal(PsBlock([])):
-                            statements_new.append(result)
+                        simplified = self.visit(stmt, ec)
+                        if not simplified.structurally_equal(PsBlock([])):
+                            statements_new.append(simplified)
                     else:
                         statements_new.append(self.visit(stmt, ec))
                 node.statements = statements_new
@@ -213,6 +213,8 @@ class EliminateBranches:
                 return PsExpression.make(PsConstant(False))
             elif intersection == outer_set:
                 return PsExpression.make(PsConstant(True))
+            else:
+                return condition
 
         except ImportError:
             return condition