[Coverity] Fix unchecked return value, NFC

[pengfei]

Summary

The ReversePredicate should have made sure the reverse predicate is
supported by target, but the check comes from early function and might
be invalid by any mistake. So it's better to double confirm it here.

[barannikov88]

llvm/lib/CodeGen/EarlyIfConversion.cpp

348

This is wrong. Assert will be removed in release mode, along with the call.

[pengfei]

Address review comment.

llvm/lib/CodeGen/EarlyIfConversion.cpp

348

Good catch, thanks!

[barannikov88]

BTW it may be worth annotating this method (and maybe others) with [[nodiscard]].

[pengfei]

In D149586#4311474, @barannikov88 wrote:

BTW it may be worth annotating this method (and maybe others) with [[nodiscard]].

Thanks for the suggestion! That sounds a good idea to me, but I took a look at current LLVM code base and didn't find an instance that using [[nodiscard]] on virtual functions. A simply googling pointed me a discussion about GCC bug.
No sure if it is the reason that we haven't used it. Anyway, let's not do it for now.

llvm/lib/CodeGen/EarlyIfConversion.cpp

 //===-- EarlyIfConversion.cpp - If-conversion on SSA form machine code ----===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 //
bool SSAIfConv::canPredicateInstrs(MachineBasicBlock *MBB) {
   }
   return true;
 }
 
 // Apply predicate to all instructions in the machine block.
 void SSAIfConv::PredicateBlock(MachineBasicBlock *MBB, bool ReversePredicate) {
   auto Condition = Cond;
   if (ReversePredicate)
-    TII->reverseBranchCondition(Condition);
+    assert(!TII->reverseBranchCondition(Condition) &&
+           "Reverse predicate is not supported");
   // Terminators don't need to be predicated as they will be removed.
   for (MachineBasicBlock::iterator I = MBB->begin(),
                                    E = MBB->getFirstTerminator();
        I != E; ++I) {
     if (I->isDebugInstr())
       continue;
     TII->PredicateInstruction(*I, Condition);
   }