[SCEV] Add a comment about invariant in howManyLessThans

lib/Analysis/ScalarEvolution.cpp

@@ -11728,6 +11728,15 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
       return getCouldNotCompute();
   }
 
+  // On all paths just preceeding, we established the following invariant:
+  //   IV can be assumed not to overflow up to and including the exiting
+  //   iteration.  We proved this in one of two ways:
+  //   1) We can show overflow doesn't occur before the exiting iteration
+  //      1a) canIVOverflowOnLT, and b) step of one
+  //   2) We can show that if overflow occurs, the loop must execute UB
+  //      before any possible exit.
+  // Note that we have not yet proved RHS invariant (in general).
+
   const SCEV *Start = IV->getStart();
 
   // Preserve pointer-typed Start/RHS to pass to isLoopEntryGuardedByCond.