Java: Fix recent perf regressions.

java/ql/src/semmle/code/java/Expr.qll

@@ -167,6 +167,7 @@ class CompileTimeConstantExpr extends Expr {
   /**
    * Gets the string value of this expression, where possible.
    */
+  pragma[nomagic]
   string getStringValue() {
     result = this.(StringLiteral).getRepresentedString()
     or

java/ql/src/semmle/code/java/dataflow/internal/DataFlowImpl.qll

@@ -2041,12 +2041,12 @@ private class PathNodeMid extends PathNode, TPathNodeMid {
     result = getSuccMid()
     or
     // a final step to a sink via zero steps means we merge the last two steps to prevent trivial-looking edges
-    exists(PathNodeMid mid |
+    exists(PathNodeMid mid, PathNodeSink sink |
       mid = getSuccMid() and
-      mid.getNode() = result.getNode() and
+      mid.getNode() = sink.getNode() and
       mid.getAp() instanceof AccessPathNil and
-      result instanceof PathNodeSink and
-      result.getConfiguration() = unbind(mid.getConfiguration())
+      sink.getConfiguration() = unbind(mid.getConfiguration()) and
+      result = sink
     )
   }