[VPlan] Remove VPInstruction::setUnderlyingInstr (NFCI).

VPInstruction doesn't rely on the underlying instruction any longer for
codegen, remove the unneeded setUnderlyingInstr.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1177,9 +1177,6 @@ class VPInstruction : public VPRecipeWithIRFlags {
   bool isFPMathOp() const;
 #endif
 
-protected:
-  void setUnderlyingInstr(Instruction *I) { setUnderlyingValue(I); }
-
 public:
   VPInstruction(unsigned Opcode, ArrayRef<VPValue *> Operands, DebugLoc DL,
                 const Twine &Name = "")

llvm/lib/Transforms/Vectorize/VPlanSLP.cpp

@@ -461,7 +461,6 @@ VPInstruction *VPlanSlp::buildGraph(ArrayRef<VPValue *> Values) {
   assert(CombinedOperands.size() > 0 && "Need more some operands");
   auto *Inst = cast<VPInstruction>(Values[0])->getUnderlyingInstr();
   auto *VPI = new VPInstruction(Opcode, CombinedOperands, Inst->getDebugLoc());
-  VPI->setUnderlyingInstr(Inst);
 
   LLVM_DEBUG(dbgs() << "Create VPInstruction " << *VPI << " "
                     << *cast<VPInstruction>(Values[0]) << "\n");