compiler: monomorphize casts first, but more carefully.

This reverts 425cd7851, which broke the use of casts to define
integer width.

Instead of it, two steps are taken:
  * First, literals are monomorphized, leading to predictable result.
  * Second, casts are monomorphized, in a top-bottom way. I.e.
    consider the expression `int64(round(x))`. If round() was visited
    first, the intermediate precision would be 32-bit, which is
    clearly undesirable. Therefore, contextual rules should take
    priority over non-contextual ones.

Fixes #1252.

artiq/compiler/transforms/cast_monomorphizer.py

@@ -11,8 +11,6 @@ class CastMonomorphizer(algorithm.Visitor):
         self.engine = engine
 
     def visit_CallT(self, node):
-        self.generic_visit(node)
-
         if (types.is_builtin(node.func.type, "int") or
                 types.is_builtin(node.func.type, "int32") or
                 types.is_builtin(node.func.type, "int64")):
@@ -30,3 +28,20 @@ class CastMonomorphizer(algorithm.Visitor):
 
                 node.args[0].type.unify(typ)
 
+        if types.is_builtin(node.func.type, "int") or \
+                types.is_builtin(node.func.type, "round"):
+            typ = node.type.find()
+            if types.is_var(typ["width"]):
+                typ["width"].unify(types.TValue(32))
+
+        self.generic_visit(node)
+
+    def visit_CoerceT(self, node):
+        if isinstance(node.value, asttyped.NumT) and \
+                builtins.is_int(node.type) and \
+                builtins.is_int(node.value.type) and \
+                not types.is_var(node.type["width"]) and \
+                types.is_var(node.value.type["width"]):
+            node.value.type.unify(node.type)
+
+        self.generic_visit(node)

artiq/compiler/transforms/int_monomorphizer.py

@@ -26,22 +26,3 @@ class IntMonomorphizer(algorithm.Visitor):
                     return
 
                 node.type["width"].unify(types.TValue(width))
-
-    def visit_CallT(self, node):
-        self.generic_visit(node)
-
-        if types.is_builtin(node.func.type, "int") or \
-                types.is_builtin(node.func.type, "round"):
-            typ = node.type.find()
-            if types.is_var(typ["width"]):
-                typ["width"].unify(types.TValue(32))
-
-    def visit_CoerceT(self, node):
-        if isinstance(node.value, asttyped.NumT) and \
-                builtins.is_int(node.type) and \
-                builtins.is_int(node.value.type) and \
-                not types.is_var(node.type["width"]) and \
-                types.is_var(node.value.type["width"]):
-            node.value.type.unify(node.type)
-
-        self.generic_visit(node)