Drop unused code-paths associated with "if cython.compiled" early (GH-3507)

This allows things to work like:
    # in pxd file
    from libc.math cimport sin

    # in py file
    if not cython.compiled:
        from math import sin  # previously failed with cython compile
           # error because it was assigning to a cdef name

This seems worthwhile because it makes it easier to write code
that re-assigns cdef names and so works in both modes.

Cython/Compiler/ParseTreeTransforms.py

@@ -630,6 +630,9 @@ class InterpretCompilerDirectives(CythonTransform):
     - Command-line arguments overriding these
     - @cython.directivename decorators
     - with cython.directivename: statements
+    - replaces "cython.compiled" with BoolNode(value=True)
+      allowing unreachable blocks to be removed at a fairly early stage
+      before cython typing rules are forced on applied
 
     This transform is responsible for interpreting these various sources
     and store the directive in two ways:
@@ -846,6 +849,16 @@ class InterpretCompilerDirectives(CythonTransform):
             directive = self.directive_names.get(node.name)
             if directive is not None:
                 node.cython_attribute = directive
+        if node.as_cython_attribute() == "compiled":
+            return ExprNodes.BoolNode(node.pos, value=True)  # replace early so unused branches can be dropped
+                # before they have a chance to cause compile-errors
+        return node
+
+    def visit_AttributeNode(self, node):
+        self.visitchildren(node)
+        if node.as_cython_attribute() == "compiled":
+            return ExprNodes.BoolNode(node.pos, value=True)  # replace early so unused branches can be dropped
+                # before they have a chance to cause compile-errors
         return node
 
     def visit_NewExprNode(self, node):
@@ -3031,9 +3044,7 @@ class TransformBuiltinMethods(EnvTransform):
     def visit_cython_attribute(self, node):
         attribute = node.as_cython_attribute()
         if attribute:
-            if attribute == u'compiled':
-                node = ExprNodes.BoolNode(node.pos, value=True)
-            elif attribute == u'__version__':
+            if attribute == u'__version__':
                 from .. import __version__ as version
                 node = ExprNodes.StringNode(node.pos, value=EncodedString(version))
             elif attribute == u'NULL':

tests/compile/cython_compiled_folding.pxd

+from libc.math cimport sin, cos, sqrt, tan, log

tests/compile/cython_compiled_folding.py

+# mode: compile
+
+# libc sin, cos and sqrt cimported in the pxd file
+
+import cython
+from cython import compiled
+
+if not cython.compiled:
+    from math import sin
+
+if cython.compiled:
+    pass
+else:
+    from math import cos
+
+if "aa" == "bb":
+    pass
+elif cython.compiled:
+    pass
+elif True:
+    from math import sqrt
+
+if "aa" == "bb":
+    pass
+elif compiled:
+    pass
+else:
+    from math import tan
+
+# log10 isn't defined in the pxd file
+from math import log10
+
+@cython.test_fail_if_path_exists("//FromImportStatNode//ImportNode")
+@cython.test_assert_path_exists("//AddNode")
+def import_log(x, y):
+    if compiled:
+        return x+y
+    else:
+        from math import log