[RISCV] Make DemandedFields::usedVTYPE() const. NFC

[craig.topper]

Summary

Noticed while reviewing D139877.

[craig.topper]

@reames FYI, if you don't write a message when you accept it doesn't go to llvm-commits.

[reames]

In D139879#3990015, @craig.topper wrote:

@reames FYI, if you don't write a message when you accept it doesn't go to llvm-commits.

Huh, never realized that. That's unfortunate.

LGTM. :)

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

 //===- RISCVInsertVSETVLI.cpp - Insert VSETVLI instructions ---------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
struct DemandedFields {
   bool VL = false;
   bool SEW = false;
   bool LMUL = false;
   bool SEWLMULRatio = false;
   bool TailPolicy = false;
   bool MaskPolicy = false;
 
   // Return true if any part of VTYPE was used
-  bool usedVTYPE() {
+  bool usedVTYPE() const {
     return SEW || LMUL || SEWLMULRatio || TailPolicy || MaskPolicy;
   }
 
   // Mark all VTYPE subfields and properties as demanded
   void demandVTYPE() {
     SEW = true;
     LMUL = true;
     SEWLMULRatio = true;