Python: Fix bad join in py/redundant-assignment

A minor thing, but still best avoided:
```
Tuple counts for RedundantAssignment::pyflakes_commented_line#bb/2@0477c7 after 14.2s:
160826   ~0%     {2} r1 = SCAN py_comments OUTPUT In.2, In.1
160826   ~0%     {3} r2 = JOIN r1 WITH project#Files::Location::hasLocationInfo_dispred#bfffff ON FIRST 1 OUTPUT Rhs.1, Lhs.1, Rhs.2 'result'
160826   ~3%     {3} r3 = JOIN r2 WITH files_10#join_rhs ON FIRST 1 OUTPUT Lhs.2 'result', Lhs.1, Rhs.1 'file'
46672419 ~4%     {4} r4 = JOIN r3 WITH Files::Location::getStartLine_dispred#bf_10#join_rhs ON FIRST 1 OUTPUT Rhs.1, Lhs.2 'file', Lhs.1, Lhs.0 'result'
3951     ~0%     {4} r5 = JOIN r4 WITH Files::Location::getFile_dispred#bf ON FIRST 2 OUTPUT Lhs.2, Lhs.3 'result', Lhs.1 'file', toLower(Lhs.2)
0        ~0%     {4} r6 = SELECT r5 ON In.3 contains "pyflakes"
0        ~0%     {2} r7 = SCAN r6 OUTPUT In.2 'file', In.1 'result'
                return r7
```

Author: tausbn

python/ql/src/Statements/RedundantAssignment.ql

@@ -70,10 +70,11 @@ predicate same_attribute(Attribute a1, Attribute a2) {
   not is_property_access(a1)
 }
 
+pragma[nomagic]
+Comment pyflakes_comment() { result.getText().toLowerCase().matches("%pyflakes%") }
+
 int pyflakes_commented_line(File file) {
-  exists(Comment c | c.getText().toLowerCase().matches("%pyflakes%") |
-    c.getLocation().hasLocationInfo(file.getAbsolutePath(), result, _, _, _)
-  )
+  pyflakes_comment().getLocation().hasLocationInfo(file.getAbsolutePath(), result, _, _, _)
 }
 
 predicate pyflakes_commented(AssignStmt assignment) {