C++: Use or instead of if

The proposition in the true branch implied the condition, so `or` is more appropriate. Also eliminated an existentially quantified variable.

cpp/ql/lib/semmle/code/cpp/rangeanalysis/SimpleRangeAnalysis.qll

@@ -111,9 +111,9 @@ float widenUpperBound(Type type, float ub) {
  * compilation units, which doesn't necessarily have a getValue() result from the extractor.
  */
 private string getValue(Expr e) {
-  if exists(e.getValue())
-  then result = e.getValue()
-  else
+  result = e.getValue()
+  or
+  not exists(e.getValue()) and
   /*
    * It should be safe to propagate the initialization value to a variable if:
    * The type of v is const, and
@@ -123,11 +123,10 @@ private string getValue(Expr e) {
    *   v is a static member variable
    */
 
-    exists(VariableAccess access, StaticStorageDurationVariable v |
+  exists(StaticStorageDurationVariable v |
     not v.getUnderlyingType().isVolatile() and
     v.getUnderlyingType().isConst() and
-      e = access and
-      v = access.getTarget() and
+    v = e.(VariableAccess).getTarget() and
     result = getValue(v.getAnAssignedValue())
   )
 }