[llvm] Fix stack frame layout diagrams in RISCVFrameLowering (NFC) (PR #190163)

[Sam Elliott via llvm-commits]

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lenary wrote:

Does `MFI.getStackSize()` still include the callee-allocated save area for register varargs? If not, can we move where that arrow points?

https://github.com/llvm/llvm-project/pull/190163