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

[Garvit Gupta via llvm-commits]

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quic-garvgupt wrote:

MFI.getStackSize() still includes the callee allocated save area for register varargs. This is the value by which sp is moved down in the stack in a reserved call frame in prologue which also includes the size for callee allocated varargs.

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