compiler: Guard cond_mapper lookup in halo hoisting

_hoist_redundant_from_conditionals iterated every halo_spot from
_filter_iter_mapper and did an unguarded cond_mapper[hs0], which raised KeyError
for a nested HaloSpot (one whose subtree contains another HaloSpot), since
_make_cond_mapper does not register nested HaloSpots as keys. Use the defensive
cond_mapper.get(hs0) (as already done in _merge_halospots); the existing
'if not conditions: continue' handles the absent-key case. Fixes #2943.

Author: ggorman

devito/passes/iet/mpi.py

@@ -88,7 +88,7 @@ def _hoist_redundant_from_conditionals(iet):
         scope = Scope(e.expr for e in FindNodes(Expression).visit(it))
 
         for hs0 in halo_spots:
-            conditions = cond_mapper[hs0]
+            conditions = cond_mapper.get(hs0)
             if not conditions:
                 continue
             condition = conditions[-1]  # Take the innermost Conditional