[llvm] Set ScalableVector stack id in proper place

Without this patch ScalableVector frame index property is used before assignment.
More precisely, let's take a look at RISCVFrameLowering::assignCalleeSavedSpillSlots.
In this function we divide callee saved registers on scalar and vector ones,
based on ScalableVector property of their frame indexes:
```
  ...
  const auto &UnmanagedCSI = getUnmanagedCSI(*MF, CSI);
  const auto &RVVCSI = getRVVCalleeSavedInfo(*MF, CSI);
  ...
```
But we assign ScalableVector property several lines below:
```
  ...
  auto storeRegToStackSlot = [&](decltype(UnmanagedCSI) CSInfo) {
    for (auto &CS : CSInfo) {
      // Insert the spill to the stack frame.
      Register Reg = CS.getReg();
      const TargetRegisterClass *RC = TRI->getMinimalPhysRegClass(Reg);
      TII.storeRegToStackSlot(MBB, MI, Reg, !MBB.isLiveIn(Reg),
                              CS.getFrameIdx(), RC, TRI, Register());
    }
  };
  storeRegToStackSlot(UnmanagedCSI);
  ...
```
Due to it, list of RVV callee saved registers will always be empty.
Currently this problem doesn't appear, but if you slightly change the code and,
for example, put some instructions between scalar and vector spills,
the resulting code will be ill formed.

Author: enoskova-sc

llvm/lib/Target/RISCV/RISCVFrameLowering.cpp

@@ -1610,6 +1610,8 @@ bool RISCVFrameLowering::assignCalleeSavedSpillSlots(
         int FrameIdx = MFI.CreateFixedSpillStackObject(Size, Offset);
         assert(FrameIdx < 0);
         CS.setFrameIdx(FrameIdx);
+        if (RISCVRegisterInfo::isRVVRegClass(RC))
+          MFI.setStackID(FrameIdx, TargetStackID::ScalableVector);
         continue;
       }
     }
@@ -1626,6 +1628,8 @@ bool RISCVFrameLowering::assignCalleeSavedSpillSlots(
     if ((unsigned)FrameIdx > MaxCSFrameIndex)
       MaxCSFrameIndex = FrameIdx;
     CS.setFrameIdx(FrameIdx);
+    if (RISCVRegisterInfo::isRVVRegClass(RC))
+      MFI.setStackID(FrameIdx, TargetStackID::ScalableVector);
   }
 
   // Allocate a fixed object that covers the full push or libcall size.