[SystemZ] Remove custom implementation of optimizeLoadInst [NFC] (#133123)

In 236f938ef, I introduced a generic version of this routine. I believe
that the SystemZ specific version of this is less general than the
generic version, and is thus unrequired. I wasn't 100% given the
difference in sub-register, multiple use and defs, but from the SystemZ
code, it looks like those cases simply don't arise?

llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp

diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
index 519baec..91a4aa9 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.cpp
@@ -670,31 +670,6 @@
     .addImm(CCValid).addImm(CCMask);
 }
 
-MachineInstr *SystemZInstrInfo::optimizeLoadInstr(MachineInstr &MI,
-                                                  const MachineRegisterInfo *MRI,
-                                                  Register &FoldAsLoadDefReg,
-                                                  MachineInstr *&DefMI) const {
-  // Check whether we can move the DefMI load, and that it only has one use.
-  DefMI = MRI->getVRegDef(FoldAsLoadDefReg);
-  assert(DefMI);
-  bool SawStore = false;
-  if (!DefMI->isSafeToMove(SawStore) || !MRI->hasOneNonDBGUse(FoldAsLoadDefReg))
-    return nullptr;
-
-  int UseOpIdx =
-      MI.findRegisterUseOperandIdx(FoldAsLoadDefReg, /*TRI=*/nullptr);
-  assert(UseOpIdx != -1 && "Expected FoldAsLoadDefReg to be used by MI.");
-
-  // Check whether we can fold the load.
-  if (MachineInstr *FoldMI =
-          foldMemoryOperand(MI, {((unsigned)UseOpIdx)}, *DefMI)) {
-    FoldAsLoadDefReg = 0;
-    return FoldMI;
-  }
-
-  return nullptr;
-}
-
 bool SystemZInstrInfo::foldImmediate(MachineInstr &UseMI, MachineInstr &DefMI,
                                      Register Reg,
                                      MachineRegisterInfo *MRI) const {

llvm/lib/Target/SystemZ/SystemZInstrInfo.h

diff --git a/llvm/lib/Target/SystemZ/SystemZInstrInfo.h b/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
index a8d282d..8b82af6 100644
--- a/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
+++ b/llvm/lib/Target/SystemZ/SystemZInstrInfo.h
@@ -258,10 +258,6 @@
                     const DebugLoc &DL, Register DstReg,
                     ArrayRef<MachineOperand> Cond, Register TrueReg,
                     Register FalseReg) const override;
-  MachineInstr *optimizeLoadInstr(MachineInstr &MI,
-                                  const MachineRegisterInfo *MRI,
-                                  Register &FoldAsLoadDefReg,
-                                  MachineInstr *&DefMI) const override;
   bool foldImmediate(MachineInstr &UseMI, MachineInstr &DefMI, Register Reg,
                      MachineRegisterInfo *MRI) const override;