[AMDGPU][Waitcnt] fix "comparison of integers of different signs" build error

[msearles]

Summary

Build error on Android; reported/fix provided by Mauro Rossi <[e-mail]> (thanks)

Fixes the following building error:

external/llvm/lib/Target/AMDGPU/SIInsertWaitcnts.cpp:1903:61:
error: comparison of integers of different signs:
'typename iterator_traits<__wrap_iter<MachineBasicBlock **> >::difference_type'
(aka 'int') and 'unsigned int' [-Werror,-Wsign-compare]
                      BlockWaitcntProcessedSet.end(), &MBB) < Count)) {
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~
1 error generated.

[kzhuravl]

LGTM

lib/Target/AMDGPU/SIInsertWaitcnts.cpp

for (ReversePostOrderTraversal<MachineFunction *>::rpo_iterator
     // at least 1 re-walk over the loop to propagate the information, even if
     // no S_WAITCNT instructions were generated.
     if (ContainingLoop && ContainingLoop->getHeader() == &MBB) {
       unsigned Count = countNumBottomBlocks(ContainingLoop);
 
       // If the loop has multiple back-edges, and so more than one "bottom"
       // basic block, we have to guarantee a re-walk over every blocks.
       if ((std::count(BlockWaitcntProcessedSet.begin(),
-                      BlockWaitcntProcessedSet.end(), &MBB) < Count)) {
+                      BlockWaitcntProcessedSet.end(), &MBB) < (int)Count)) {
         BlockWaitcntBracketsMap[&MBB]->setRevisitLoop(true);
         LLVM_DEBUG(dbgs() << "set-revisit1: Block"
                           << ContainingLoop->getHeader()->getNumber() << '\n';);
       }
     }
 
     // Walk over the instructions.
     insertWaitcntInBlock(MF, MBB);