Revert "ssa: Associate ExprNode with a variable"

This reverts commit 75e57dd.

Author: asgerf

shared/ssa/codeql/ssa/Ssa.qll

@@ -1680,12 +1680,7 @@ module Make<
     cached
     private newtype TNode =
       TWriteDefSource(WriteDefinition def) { DfInput::ssaDefHasSource(def) } or
-      TExprNode(DfInput::Expr e, SourceVariable v, Boolean isPost) {
-        exists(Definition def |
-          def.getSourceVariable() = v and
-          e = DfInput::getARead(def)
-        )
-      } or
+      TExprNode(DfInput::Expr e, Boolean isPost) { e = DfInput::getARead(_) } or
       TSsaDefinitionNode(DefinitionExt def) {
         not phiHasUniqNextNode(def) and
         if DfInput::includeWriteDefsInFlowStep()
@@ -1735,9 +1730,8 @@ module Make<
     abstract private class ExprNodePreOrPostImpl extends NodeImpl, TExprNode {
       DfInput::Expr e;
       boolean isPost;
-      SourceVariable v_;
 
-      ExprNodePreOrPostImpl() { this = TExprNode(e, v_, isPost) }
+      ExprNodePreOrPostImpl() { this = TExprNode(e, isPost) }
 
       /** Gets the underlying expression. */
       DfInput::Expr getExpr() { result = e }
@@ -1748,9 +1742,6 @@ module Make<
           result = bb.getNode(i).getLocation()
         )
       }
-
-      /** Gets the variable accessed at this expression. */
-      SourceVariable getSourceVariable() { result = v_ }
     }
 
     final class ExprNodePreOrPost = ExprNodePreOrPostImpl;
@@ -1769,7 +1760,7 @@ module Make<
       ExprPostUpdateNodeImpl() { isPost = true }
 
       /** Gets the pre-update expression node. */
-      ExprNode getPreUpdateNode() { result = TExprNode(e, _, false) }
+      ExprNode getPreUpdateNode() { result = TExprNode(e, false) }
 
       override string toString() { result = e.toString() + " [postupdate]" }
     }
@@ -1779,6 +1770,7 @@ module Make<
     private class ReadNodeImpl extends ExprNodeImpl {
       private BasicBlock bb_;
       private int i_;
+      private SourceVariable v_;
 
       ReadNodeImpl() {
         variableRead(bb_, i_, v_, true) and