[llvm] [RISC-V] Fix incorrect epilogue_begin (PR #120623)

[Mikhail Gudim via llvm-commits]

mgudim wrote:

It seems like in general we cannot guarantee that prolog / epilog code is a contiguous region since instructions can be moved by optimisation. What is the solution in that case?

Not saying we should address this in the PR, just wondering.

https://github.com/llvm/llvm-project/pull/120623