[3/15][Clang][RISCV][NFC] Clarify edge cases of RVVIntrinsic::getSupportedMaskedPolicies for clarity

[eopXD]

Summary

This is the 3rd commit of a patch-set that aims to change the default policy
for RVV intrinsics from TAMU to TAMA.

Please refer to the cover letter in the 1st commit (D141573) for an
overview.

[eopXD]

Bump CI

[eopXD]

Bump CI

[eopXD]

Rebase to latest main.

[kito-cheng]

LGTM

clang/lib/Support/RISCVVIntrinsicUtils.cpp

 //===- RISCVVIntrinsicUtils.cpp - RISC-V Vector Intrinsic Utils -*- C++ -*-===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/Support/RISCVVIntrinsicUtils.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/ADT/Twine.h"
+#include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 #include <numeric>
 #include <optional>
 
 using namespace llvm;
 
 namespace clang {
 namespace RISCV {
if (HasTailPolicy && HasMaskPolicy)
     return {Policy(Policy::PolicyType::Undisturbed,
                    Policy::PolicyType::Agnostic), // TUMA
             Policy(Policy::PolicyType::Agnostic,
                    Policy::PolicyType::Agnostic), // TAMA
             Policy(Policy::PolicyType::Undisturbed,
                    Policy::PolicyType::Undisturbed), // TUMU
             Policy(Policy::PolicyType::Agnostic,
                    Policy::PolicyType::Undisturbed)}; // TAMU
-
-  if (HasTailPolicy)
+  if (HasTailPolicy && !HasMaskPolicy)
     return {Policy(Policy::PolicyType::Undisturbed,
                    Policy::PolicyType::Agnostic, true), // TUM
             Policy(Policy::PolicyType::Agnostic, Policy::PolicyType::Agnostic,
                    true)}; // TAM
-
+  if (!HasTailPolicy && HasMaskPolicy)
     return {
-      Policy(Policy::PolicyType::Omit, Policy::PolicyType::Agnostic),     // MA
-      Policy(Policy::PolicyType::Omit, Policy::PolicyType::Undisturbed)}; // MU
+        Policy(Policy::PolicyType::Omit, Policy::PolicyType::Agnostic), // MA
+        Policy(Policy::PolicyType::Omit,
+               Policy::PolicyType::Undisturbed)}; // MU
+  llvm_unreachable("An RVV instruction should not be without both tail policy "
+                   "and mask policy");
 }
 
 void RVVIntrinsic::updateNamesAndPolicy(bool IsMasked, bool HasPolicy,
                                         std::string &Name,
                                         std::string &BuiltinName,
                                         std::string &OverloadedName,
                                         Policy &PolicyAttrs) {