Fix warning: comparison of integer expressions of different signedness

[lbenes]

Summary

This patch resolves the Wsign-compare warning that I observed on armv7l and x86 with both gcc and clang.

[pengfei]

LG. Thanks for fixing it.
(do you want me to land this?)

llvm/lib/Target/X86/X86TileConfig.cpp

for (auto &R : {Shape.getRow()->getReg(), Shape.getCol()->getReg()}) {
           LIS.InsertMachineInstrInMaps(*NewMI);
         } else {
           unsigned SubIdx = IsRow ? X86::sub_8bit : X86::sub_16bit;
           unsigned RegSize = TRI->getRegSizeInBits(*MRI.getRegClass(R));
           if ((IsRow && RegSize == 8) || (!IsRow && RegSize == 16))
             SubIdx = 0;
           auto Iter = DefMI.getIterator();
           if (&MBB == &MF.front() &&
-              std::distance(MBB.instr_begin(), Iter) < ConstPos)
+              (unsigned)std::distance(MBB.instr_begin(), Iter) < ConstPos)
             Iter = ConstMI->getIterator();
           NewMI = addFrameReference(
                       BuildMI(MBB, ++Iter, DL,
                               TII->get(IsRow ? X86::MOV8mr : X86::MOV16mr)),
                       SS, Offset)
                       .addReg(R, 0, SubIdx);
           SlotIndex SIdx = LIS.InsertMachineInstrInMaps(*NewMI);
           LIS.extendToIndices(LIS.getInterval(R), {SIdx.getRegSlot()});