Python: Fix bad join in RegExpBackRef::getGroup

Although this wasn't (as far as I know) causing any performance issues,
it was making the join-order badness report quite noisy, and so I
figured it was worth fixing.

Before:
```
Tuple counts for RegexTreeView::RegExpBackRef::getGroup#dispred#f0820431#ff/2@d3441d0b after 84ms:
1501195 ~3%     {2} r1 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT Rhs.1 'result', Lhs.1 'result'
149     ~0%     {5} r2 = JOIN r1 WITH RegexTreeView::RegExpBackRef#class#31aac2a7#ffff ON FIRST 1 OUTPUT Rhs.1, Rhs.2, Rhs.3, Lhs.1 'result', Lhs.0 'this'
149     ~1%     {3} r3 = JOIN r2 WITH regex::RegexString::numbered_backreference#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.3 'result', Rhs.3, Lhs.4 'this'
4       ~0%     {2} r4 = JOIN r3 WITH RegexTreeView::RegExpGroup::getNumber#dispred#f0820431#ff ON FIRST 2 OUTPUT Lhs.2 'this', Lhs.0 'result'

1501195 ~3%     {2} r5 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff_10#join_rhs ON FIRST 1 OUTPUT Lhs.1 'result', Rhs.1 'result'
42526   ~0%     {5} r6 = JOIN r5 WITH RegexTreeView::RegExpGroup#31aac2a7#ffff ON FIRST 1 OUTPUT Lhs.1 'this', Lhs.0 'result', Rhs.1, Rhs.2, Rhs.3
22      ~0%     {8} r7 = JOIN r6 WITH RegexTreeView::RegExpBackRef#class#31aac2a7#ffff ON FIRST 1 OUTPUT Lhs.2, Lhs.3, Lhs.4, Lhs.1 'result', Lhs.0 'this', Rhs.1, Rhs.2, Rhs.3
0       ~0%     {6} r8 = JOIN r7 WITH regex::RegexString::getGroupName#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.5, Lhs.6, Lhs.7, Rhs.3, Lhs.3 'result', Lhs.4 'this'
0       ~0%     {2} r9 = JOIN r8 WITH regex::RegexString::named_backreference#dispred#f0820431#ffff ON FIRST 4 OUTPUT Lhs.5 'this', Lhs.4 'result'

4       ~0%     {2} r10 = r4 UNION r9
                return r10
```

In this case I opted for a classical solution: tying together the
literal and number (or name) part of the backreference in order to
encourage a two-column join.

After:
```
Tuple counts for RegexTreeView::RegExpBackRef::getGroup#dispred#f0820431#ff/2@b0cc4d5n after 0ms:
898  ~1%     {3} r1 = JOIN RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff WITH RegexTreeView::RegExpGroup::getNumber#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Rhs.1, Lhs.0 'result'
4    ~0%     {2} r2 = JOIN r1 WITH RegexTreeView::RegExpBackRef::hasLiteralAndNumber#f0820431#fff_120#join_rhs ON FIRST 2 OUTPUT Rhs.2 'this', Lhs.2 'result'

1110 ~0%     {5} r3 = JOIN RegexTreeView::RegExpGroup#31aac2a7#ffff WITH RegexTreeView::RegExpTerm::getLiteral#dispred#f0820431#ff ON FIRST 1 OUTPUT Lhs.1, Lhs.2, Lhs.3, Lhs.0 'result', Rhs.1
146  ~0%     {3} r4 = JOIN r3 WITH regex::RegexString::getGroupName#dispred#f0820431#ffff ON FIRST 3 OUTPUT Lhs.4, Rhs.3, Lhs.3 'result'
0    ~0%     {2} r5 = JOIN r4 WITH RegexTreeView::RegExpBackRef::hasLiteralAndName#f0820431#fff_120#join_rhs ON FIRST 2 OUTPUT Rhs.2 'this', Lhs.2 'result'

4    ~0%     {2} r6 = r2 UNION r5
            return r6
```

Author: tausbn

python/ql/lib/semmle/python/RegexTreeView.qll

@@ -1000,11 +1000,22 @@ class RegExpBackRef extends RegExpTerm, TRegExpBackRef {
 
   /** Gets the capture group this back reference refers to. */
   RegExpGroup getGroup() {
-    result.getLiteral() = this.getLiteral() and
-    (
-      result.getNumber() = this.getNumber() or
-      result.getName() = this.getName()
-    )
+    this.hasLiteralAndNumber(result.getLiteral(), result.getNumber()) or
+    this.hasLiteralAndName(result.getLiteral(), result.getName())
+  }
+
+  /** Join-order helper for `getGroup`. */
+  pragma[nomagic]
+  private predicate hasLiteralAndNumber(RegExpLiteral literal, int number) {
+    literal = this.getLiteral() and
+    number = this.getNumber()
+  }
+
+  /** Join-order helper for `getGroup`. */
+  pragma[nomagic]
+  private predicate hasLiteralAndName(RegExpLiteral literal, string name) {
+    literal = this.getLiteral() and
+    name = this.getName()
   }
 
   override RegExpTerm getChild(int i) { none() }