[mte] fix runOnFunction return value falsely suggesting function was modified.

If NumInterestingAllocas == 0 we do not add any instrumentation or
padding in the previous loop.

Reviewed By: eugenis

Differential Revision: https://reviews.llvm.org/D118961

llvm/lib/Target/AArch64/AArch64StackTagging.cpp

@@ -605,7 +605,7 @@ bool AArch64StackTagging::runOnFunction(Function &Fn) {
   }
 
   if (NumInterestingAllocas == 0)
-    return true;
+    return false;
 
   std::unique_ptr<DominatorTree> DeleteDT;
   DominatorTree *DT = nullptr;