[LoongArch] Add some comments for expand pseudo-inst pass. NFC

llvm/lib/Target/LoongArch/LoongArchExpandPseudoInsts.cpp

bool LoongArchPreRAExpandPseudo::expandPcalau12iInstPair(
 
   MI.eraseFromParent();
   return true;
 }
 
 bool LoongArchPreRAExpandPseudo::expandLoadAddressPcrel(
     MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI,
     MachineBasicBlock::iterator &NextMBBI) {
+  // Code Sequence:
+  // pcalau12i $rd, %pc_hi20(sym)
+  // addi.w/d $rd, $rd, %pc_lo12(sym)
   MachineFunction *MF = MBB.getParent();
   const auto &STI = MF->getSubtarget<LoongArchSubtarget>();
   unsigned SecondOpcode = STI.is64Bit() ? LoongArch::ADDI_D : LoongArch::ADDI_W;
   return expandPcalau12iInstPair(MBB, MBBI, NextMBBI, LoongArchII::MO_PCREL_HI,
                                  SecondOpcode, LoongArchII::MO_PCREL_LO);
 }
 
 bool LoongArchPreRAExpandPseudo::expandLoadAddressGot(
     MachineBasicBlock &MBB, MachineBasicBlock::iterator MBBI,
     MachineBasicBlock::iterator &NextMBBI) {
+  // Code Sequence:
+  // pcalau12i $rd, %got_pc_hi20(sym)
+  // ld.w/d $rd, $rd, %got_pc_lo12(sym)
   MachineFunction *MF = MBB.getParent();
   const auto &STI = MF->getSubtarget<LoongArchSubtarget>();
   unsigned SecondOpcode = STI.is64Bit() ? LoongArch::LD_D : LoongArch::LD_W;
   return expandPcalau12iInstPair(MBB, MBBI, NextMBBI, LoongArchII::MO_GOT_PC_HI,
                                  SecondOpcode, LoongArchII::MO_GOT_PC_LO);
 }
 
 } // end namespace