[RISCV] Duplicate pseudo expansion comment to RISCVMCCodeEmitter

Follow-on from D77443. Although we're not fixing any of these
pseudo-instructions, the potential for them to be out of sync still
exists.

Author: jrtc27

llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp

@@ -189,6 +189,9 @@ void RISCVMCCodeEmitter::encodeInstruction(const MCInst &MI, raw_ostream &OS,
   // Get byte count of instruction.
   unsigned Size = Desc.getSize();
 
+  // RISCVInstrInfo::getInstSizeInBytes hard-codes the number of expanded
+  // instructions for each pseudo, and must be updated when adding new pseudos
+  // or changing existing ones.
   if (MI.getOpcode() == RISCV::PseudoCALLReg ||
       MI.getOpcode() == RISCV::PseudoCALL ||
       MI.getOpcode() == RISCV::PseudoTAIL ||