[RISCV] Remove shadow register list passed to AllocateReg when allocating FP registers

[craig.topper]

Summary

The _F and _D registers are already sub/super registers. When one gets allocated all its aliases are already marked as allocated. We don't need to explicitly shadow it too.

I believe shadow is for calling conventions like 64-bit Windows on X86. Where have rules like this

CCIfType<[i32], CCAssignToRegWithShadow<[ECX , EDX , R8D , R9D ],              
                                         [XMM0, XMM1, XMM2, XMM3]>>

For that calling convention the argument number determines which register is used regardless of how many scalars or vectors came before it.

Removing this removes a question I had in D90738.

[HsiangKai]

LGTM.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

if (ArgFlags.isSplitEnd() && PendingLocs.size() <= 2) {
     PendingArgFlags.clear();
     return CC_RISCVAssign2XLen(XLen, State, VA, AF, ValNo, ValVT, LocVT,
                                ArgFlags);
   }
 
   // Allocate to a register if possible, or else a stack slot.
   Register Reg;
   if (ValVT == MVT::f32 && !UseGPRForF32)
-    Reg = State.AllocateReg(ArgFPR32s, ArgFPR64s);
+    Reg = State.AllocateReg(ArgFPR32s);
   else if (ValVT == MVT::f64 && !UseGPRForF64)
-    Reg = State.AllocateReg(ArgFPR64s, ArgFPR32s);
+    Reg = State.AllocateReg(ArgFPR64s);
   else
     Reg = State.AllocateReg(ArgGPRs);
   unsigned StackOffset =
       Reg ? 0 : State.AllocateStack(XLen / 8, Align(XLen / 8));
 
   // If we reach this point and PendingLocs is non-empty, we must be at the
   // end of a split argument that must be passed indirectly.
   if (!PendingLocs.empty()) {