[llvm][RISCV] Set ScalableVector stack id in proper place (#117862)

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.