Avoid side effects of additional Interpret() during iteration

... which could alter results if this formula cell again depends
on other formula cells in the circular reference(s).

Change-Id: I2b56902a71c0f16f9777507402ea004ae609bfbc
Reviewed-on: https://gerrit.libreoffice.org/56182Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>

sc/source/core/data/formulacell.cxx

@@ -2598,6 +2598,11 @@ bool ScFormulaCell::IsMultilineResult()
 
 bool ScFormulaCell::NeedsInterpret() const
 {
+    if (bIsIterCell)
+        // Shortcut to force return of current value and not enter Interpret()
+        // as we're looping over all iteration cells.
+        return false;
+
     if (!IsDirtyOrInTableOpDirty())
         return false;