C#: Account for split SSA definitions in guards library

On 03e69e9, I updated the guards library to account
for control flow graph splitting. However, the logic that relates SSA qualifiers for
the guard and the guarded expression was not updated accordingly.

Author: hvitved

csharp/ql/src/semmle/code/csharp/controlflow/Guards.qll

@@ -285,7 +285,7 @@ class AccessOrCallExpr extends Expr {
   Declaration getTarget() { result = target }
 
   /**
-   * Gets the (non-trivial) SSA definition corresponding to the longest
+   * Gets a (non-trivial) SSA definition corresponding to the longest
    * qualifier chain of this expression, if any.
    *
    * This includes the case where this expression is itself an access to an
@@ -299,25 +299,23 @@ class AccessOrCallExpr extends Expr {
    * x.Foo().Bar(); // SSA qualifier: SSA definition for `x`
    * x;             // SSA qualifier: SSA definition for `x`
    * ```
+   *
+   * An expression can have more than one SSA qualifier in the presence
+   * of control flow splitting.
    */
-  Ssa::Definition getSsaQualifier() { result = getSsaQualifier(this) }
-
-  /**
-   * Holds if this expression has an SSA qualifier.
-   */
-  predicate hasSsaQualifier() { exists(this.getSsaQualifier()) }
+  Ssa::Definition getAnSsaQualifier() { result = getAnSsaQualifier(this) }
 }
 
 private Declaration getDeclarationTarget(Expr e) {
   e = any(AssignableRead ar | result = ar.getTarget()) or
   result = e.(Call).getTarget()
 }
 
-private Ssa::Definition getSsaQualifier(Expr e) {
+private Ssa::Definition getAnSsaQualifier(Expr e) {
   e = getATrackedRead(result)
   or
   not e = getATrackedRead(_) and
-  result = getSsaQualifier(e.(QualifiableExpr).getQualifier())
+  result = getAnSsaQualifier(e.(QualifiableExpr).getQualifier())
 }
 
 private AssignableRead getATrackedRead(Ssa::Definition def) {
@@ -688,10 +686,9 @@ module Internal {
     predicate isGuardedBy(AccessOrCallExpr guarded, Guard g, AccessOrCallExpr sub, AbstractValue v) {
       isGuardedBy1(guarded, g, sub, v) and
       sub = g.getAChildExpr*() and
-      (
-        not guarded.hasSsaQualifier() and not sub.hasSsaQualifier()
-        or
-        guarded.getSsaQualifier() = sub.getSsaQualifier()
+      forall(Ssa::Definition def |
+        def = sub.getAnSsaQualifier() |
+        def = guarded.getAnSsaQualifier()
       )
     }
   }

csharp/ql/test/library-tests/controlflow/guards/BooleanGuardedExpr.expected

@@ -81,4 +81,3 @@
 | Splitting.cs:119:13:119:13 | access to parameter o | Splitting.cs:116:22:116:30 | ... != ... | Splitting.cs:116:22:116:22 | access to parameter o | true |
 | Splitting.cs:120:16:120:16 | access to parameter o | Splitting.cs:116:22:116:30 | ... != ... | Splitting.cs:116:22:116:22 | access to parameter o | true |
 | Splitting.cs:132:25:132:25 | access to parameter b | Splitting.cs:130:21:130:21 | access to parameter b | Splitting.cs:130:21:130:21 | access to parameter b | false |
-| Splitting.cs:133:17:133:17 | access to local variable o | Splitting.cs:128:17:128:25 | ... != ... | Splitting.cs:128:17:128:17 | access to local variable o | true |

csharp/ql/test/library-tests/controlflow/guards/GuardedExpr.expected

@@ -192,5 +192,3 @@
 | Splitting.cs:120:16:120:16 | access to parameter o | Splitting.cs:116:22:116:22 | access to parameter o | Splitting.cs:116:22:116:22 | access to parameter o | non-null |
 | Splitting.cs:120:16:120:16 | access to parameter o | Splitting.cs:116:22:116:30 | ... != ... | Splitting.cs:116:22:116:22 | access to parameter o | true |
 | Splitting.cs:132:25:132:25 | access to parameter b | Splitting.cs:130:21:130:21 | access to parameter b | Splitting.cs:130:21:130:21 | access to parameter b | false |
-| Splitting.cs:133:17:133:17 | access to local variable o | Splitting.cs:128:17:128:17 | access to local variable o | Splitting.cs:128:17:128:17 | access to local variable o | non-null |
-| Splitting.cs:133:17:133:17 | access to local variable o | Splitting.cs:128:17:128:25 | ... != ... | Splitting.cs:128:17:128:17 | access to local variable o | true |

csharp/ql/test/library-tests/controlflow/guards/NullGuardedExpr.expected

@@ -53,4 +53,3 @@
 | Splitting.cs:117:9:117:9 | access to parameter o |
 | Splitting.cs:119:13:119:13 | access to parameter o |
 | Splitting.cs:120:16:120:16 | access to parameter o |
-| Splitting.cs:133:17:133:17 | access to local variable o |