[PowerPC] remove XXSWAPD after vector splat immediate

[tingwang]

Summary

Redundant XXSWAPD has been observed on little-endian in D138883 test case. Remove those associated with vector splat instructions.

[shchenz]

LGTM. Thanks

llvm/lib/Target/PowerPC/PPCMIPeephole.cpp

for (MachineInstr &MI : MBB) {
           }
           // Splat fed by another splat - switch the output of the first
           // and remove the second.
           DefMI->getOperand(0).setReg(MI.getOperand(0).getReg());
           LLVM_DEBUG(dbgs() << "Removing redundant splat: ");
           LLVM_DEBUG(MI.dump());
         } else if (Immed == 2 &&
                    (DefOpc == PPC::VSPLTB || DefOpc == PPC::VSPLTH ||
-                    DefOpc == PPC::VSPLTW || DefOpc == PPC::XXSPLTW)) {
+                    DefOpc == PPC::VSPLTW || DefOpc == PPC::XXSPLTW ||
+                    DefOpc == PPC::VSPLTISB || DefOpc == PPC::VSPLTISH ||
+                    DefOpc == PPC::VSPLTISW)) {
           // Swap of various vector splats, convert to copy.
           ToErase = &MI;
           Simplified = true;
-          LLVM_DEBUG(dbgs() << "Optimizing swap(vsplt[b|h|w]|xxspltw) => "
-                               "copy(vsplt[b|h|w]|xxspltw): ");
+          LLVM_DEBUG(dbgs() << "Optimizing swap(vsplt(is)?[b|h|w]|xxspltw) => "
+                               "copy(vsplt(is)?[b|h|w]|xxspltw): ");
           LLVM_DEBUG(MI.dump());
           BuildMI(MBB, &MI, MI.getDebugLoc(), TII->get(PPC::COPY),
                   MI.getOperand(0).getReg())
               .add(MI.getOperand(1));
         } else if ((Immed == 0 || Immed == 3 || Immed == 2) &&
                    TII->isLoadFromConstantPool(DefMI)) {
           const Constant *C = TII->getConstantFromConstantPool(DefMI);
           if (C && C->getType()->isVectorTy() && C->getSplatValue()) {