[LoongArch] Support fastcc and treat it as ccc

[xen0n]

Summary

As explained in D68559 the fastcc calling convention may be requested
under certain conditions, hence the need for supporting it. But unlike
RISCV we actually treat it exactly like ccc, without actually inventing
any performance hack right here. And CSKY does the same thing.

This is going to fix a few more test cases with native LoongArch builds.

[SixWeining]

LGTM. Thanks.

llvm/lib/Target/LoongArch/LoongArchISelLowering.cpp

SDValue LoongArchTargetLowering::LowerFormalArguments(
     SelectionDAG &DAG, SmallVectorImpl<SDValue> &InVals) const {
 
   MachineFunction &MF = DAG.getMachineFunction();
 
   switch (CallConv) {
   default:
     llvm_unreachable("Unsupported calling convention");
   case CallingConv::C:
+  case CallingConv::Fast:
     break;
   }
 
   EVT PtrVT = getPointerTy(DAG.getDataLayout());
   MVT GRLenVT = Subtarget.getGRLenVT();
   unsigned GRLenInBytes = Subtarget.getGRLen() / 8;
   // Used with varargs to acumulate store chains.
   std::vector<SDValue> OutChains;