Add comments about the reason to remove x7.

Author: Yeting Kuo

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -18884,6 +18884,7 @@ ArrayRef<MCPhysReg> RISCV::getArgGPRs(const RISCVABI::ABI ABI) {
 static ArrayRef<MCPhysReg> getFastCCArgGPRs(const RISCVABI::ABI ABI) {
   // The GPRs used for passing arguments in the FastCC, X5 and X6 might be used
   // for save-restore libcall, so we don't use them.
+  // Don't use X7 for fastcc, since Zicfilp uses X7 as the label register.
   static const MCPhysReg FastCCIGPRs[] = {
       RISCV::X10, RISCV::X11, RISCV::X12, RISCV::X13, RISCV::X14,
       RISCV::X15, RISCV::X16, RISCV::X17, RISCV::X28, RISCV::X29,