[RISCV][GISEL] Add support for lowerFormalArguments that contain scalable vector types

Scalable vector types from LLVM IR can be lowered to scalable vector
types in MIR according to the RISCV::CC_RISCV function.

Author: michaelmaitland

llvm/lib/CodeGen/LowLevelType.cpp

@@ -17,7 +17,7 @@ using namespace llvm;
 
 LLT::LLT(MVT VT) {
   if (VT.isVector()) {
-    bool asVector = VT.getVectorMinNumElements() > 1;
+    bool asVector = VT.getVectorMinNumElements() > 1 || VT.isScalableVector();
     init(/*IsPointer=*/false, asVector, /*IsScalar=*/!asVector,
          VT.getVectorElementCount(), VT.getVectorElementType().getSizeInBits(),
          /*AddressSpace=*/0);

llvm/lib/Target/RISCV/GISel/RISCVCallLowering.cpp

@@ -14,6 +14,7 @@
 
 #include "RISCVCallLowering.h"
 #include "RISCVISelLowering.h"
+#include "RISCVMachineFunctionInfo.h"
 #include "RISCVSubtarget.h"
 #include "llvm/CodeGen/Analysis.h"
 #include "llvm/CodeGen/GlobalISel/MachineIRBuilder.h"
@@ -181,6 +182,9 @@ struct RISCVIncomingValueAssigner : public CallLowering::IncomingValueAssigner {
     const DataLayout &DL = MF.getDataLayout();
     const RISCVSubtarget &Subtarget = MF.getSubtarget<RISCVSubtarget>();
 
+    if (LocVT.isScalableVector())
+      MF.getInfo<RISCVMachineFunctionInfo>()->setIsVectorCall();
+
     return RISCVAssignFn(DL, Subtarget.getTargetABI(), ValNo, ValVT, LocVT,
                          LocInfo, Flags, State, /*IsFixed=*/true, IsRet,
                          Info.Ty, *Subtarget.getTargetLowering(),
@@ -303,6 +307,9 @@ static bool isSupportedArgumentType(Type *T, const RISCVSubtarget &Subtarget) {
     return true;
   if (T->isPointerTy())
     return true;
+  // TODO: Support fixed vector types.
+  if (T->isVectorTy() && T->isScalableTy() && Subtarget.hasVInstructions())
+    return true;
   return false;
 }