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

@@ -47,6 +47,9 @@ class EliminateBranches:
 
     This pass will attempt to evaluate branch conditions within their context in the AST, and replace
     conditionals by either their then- or their else-block if the branch is unequivocal.
+
+    If islpy is installed, this pass will incorporate information about the iteration regions
+    of enclosing loops and enclosing conditionals into its analysis.
     """
 
     def __init__(self, ctx: KernelCreationContext) -> None: