C++: make IR BarrierGuard::checks match AST

Author: rdmarsh2

cpp/ql/src/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll

@@ -699,13 +699,20 @@ predicate localExprFlow(Expr e1, Expr e2) { localFlow(exprNode(e1), exprNode(e2)
  */
 class BarrierGuard extends IRGuardCondition {
   /** Override this predicate to hold if this guard validates `instr` upon evaluating to `b`. */
-  abstract predicate checks(Instruction instr, boolean b);
+  predicate checksInstr(Instruction instr, boolean b) { none() }
+
+  /** Override this predicate to hold if this guard validates `expr` upon evaluating to `b`. */
+  predicate checks(Expr e, boolean b) { none() }
 
   /** Gets a node guarded by this guard. */
   final Node getAGuardedNode() {
     exists(ValueNumber value, boolean edge |
+      (
+        this.checksInstr(value.getAnInstruction(), edge)
+        or
+        this.checks(value.getAnInstruction().getConvertedResultExpression(), edge)
+      ) and
       result.asInstruction() = value.getAnInstruction() and
-      this.checks(value.getAnInstruction(), edge) and
       this.controls(result.asInstruction().getBlock(), edge)
     )
   }

cpp/ql/test/library-tests/dataflow/dataflow-tests/IRDataflowTestCommon.qll

@@ -10,7 +10,7 @@ import semmle.code.cpp.ir.IR
 class TestBarrierGuard extends DataFlow::BarrierGuard {
   TestBarrierGuard() { this.(CallInstruction).getStaticCallTarget().getName() = "guarded" }
 
-  override predicate checks(Instruction checked, boolean isTrue) {
+  override predicate checksInstr(Instruction checked, boolean isTrue) {
     checked = this.(CallInstruction).getPositionalArgument(0) and
     isTrue = true
   }