C#: Do not use the SSA definition expression for sign of increment and decrement (only use the specialized code, which is already present).

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/dataflow/internal/rangeanalysis/SignAnalysisSpecific.qll

@@ -125,7 +125,9 @@ private module Impl {
   SsaExplicitWrite getExplicitSsaAssignment(SsaExplicitWrite v) { result = v }
 
   /** Returns the assignment of the variable update `def`. */
-  ExprNode getExprFromSsaAssignment(SsaExplicitWrite def) { result.getExpr() = def.getValue() }
+  ExprNode getExprFromSsaAssignment(SsaExplicitWrite def) {
+    result.getExpr() = def.getValue() and not def.getDefiningExpr() instanceof MutatorOperation
+  }
 
   /** Holds if `def` can have any sign. */
   predicate explicitSsaDefWithAnySign(SsaExplicitWrite def) {