compiler: rename named unused global vars to "_"

So that (*codegen).Visit is able to omit code generation for these
unused global vars. The most tricky part is to detect unused global
variables, it is done in several steps:
1. Collect the set of named used/unused global vars.
2. Collect the set of globally declared expressions that contain
function calls.
3. Pick up global vars from the set made at step 2.
4. Traverse used functions and puck up those global vars that are used
from these functions.
5. Rename all globals that are presented in the set made at step 1
but are not presented in the set made on step 3 or step 4.

pkg/compiler/testdata/foo/foo.go

@@ -5,7 +5,7 @@ func NewBar() int {
 	return 10
 }
 
-// Dummy is dummy constant.
+// Dummy is dummy variable.
 var Dummy = 1
 
 // Foo is a type.