Revert "Revert "[NFC][llvm] Make the contructors of `ElementCount` private.""

Was reverted because MLIR/Flang builds were broken, these APIs have been
fixed in the meantime.
diff --git a/llvm/lib/Analysis/VFABIDemangling.cpp b/llvm/lib/Analysis/VFABIDemangling.cpp
index 0192a21..56155b2 100644
--- a/llvm/lib/Analysis/VFABIDemangling.cpp
+++ b/llvm/lib/Analysis/VFABIDemangling.cpp
@@ -310,7 +310,7 @@
     if (auto *VTy = dyn_cast<VectorType>(Ty))
       return VTy->getElementCount();
 
-  return ElementCount(/*Min=*/1, /*Scalable=*/false);
+  return ElementCount::getFixed(/*Min=*/1);
 }
 } // namespace