Expression: Remove logical literals in and/or ops during simplify

The current implementation would only resolve .and./.or. operations
if all arguments are logical literals. Insterad, we can evaluate
`.true. and a` => `.true.` and `.false. and a` => `a`, and vice
versa for `or` operations.

loki/expression/symbolic.py

@@ -585,21 +585,25 @@ def map_comparison(self, expr, *args, **kwargs):
 
     def map_logical_and(self, expr, *args, **kwargs):
         children = tuple(self.rec(child, *args, **kwargs) for child in expr.children)
-        if all(isinstance(c, sym.LogicLiteral) for c in children):
-            if all(c == 'True' for c in children):
-                return sym.LogicLiteral('True')
-            return sym.LogicLiteral('False')
+        if self.enabled_simplifications & Simplification.LogicEvaluation:
+            if any(c == 'False' for c in children):
+                return sym.LogicLiteral('False')
+            if any(c == 'True' for c in children):
+                # Trim all literals and return .true. if all were .true.
+                children = tuple(c for c in children if not c == 'True')
 
-        return sym.LogicalAnd(children)
+        return sym.LogicalAnd(children) if len(children) > 0 else sym.LogicLiteral('True')
 
     def map_logical_or(self, expr, *args, **kwargs):
         children = tuple(self.rec(child, *args, **kwargs) for child in expr.children)
-        if all(isinstance(c, sym.LogicLiteral) for c in children):
+        if self.enabled_simplifications & Simplification.LogicEvaluation:
             if any(c == 'True' for c in children):
                 return sym.LogicLiteral('True')
-            return sym.LogicLiteral('False')
+            if any(c == 'False' for c in children):
+                # Trim all literals and return .false. if all were .false.
+                children = tuple(c for c in children if not c == 'False')
 
-        return sym.LogicalOr(children)
+        return sym.LogicalOr(children) if len(children) > 0 else sym.LogicLiteral('False')
 
 
 def simplify(expr, enabled_simplifications=Simplification.ALL):

loki/expression/tests/test_symbolic.py

@@ -197,6 +197,10 @@ def test_simplify_collect_coefficients(source, ref):
     ('.false. .or. .false.', 'False'),
     ('2 == 1 .and. 1 == 1', 'False'),
     ('2 == 1 .or. 1 == 1', 'True'),
+    ('.true. .or. a', 'True'),
+    ('.false. .or. a', 'a'),
+    ('.false. .and. a', 'False'),
+    ('.true. .and. a', 'a'),
 ])
 def test_simplify_logic_evaluation(source, ref):
     scope = Scope()