[InstCombine] Adjust fixpoint error message (NFC)

Add a hint to use the no-verify-fixpoint option.

Author: nikic

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -5428,7 +5428,8 @@ static bool combineInstructionsOverFunction(
     if (Iteration > Opts.MaxIterations) {
       report_fatal_error(
           "Instruction Combining did not reach a fixpoint after " +
-              Twine(Opts.MaxIterations) + " iterations",
+              Twine(Opts.MaxIterations) + " iterations. " +
+              "Use 'instcombine<no-verify-fixpoint>' to suppress this error.",
           /*GenCrashDiag=*/false);
     }
   }