[llvm] [NFC][RISCV] Refactor allocation of the stack space (PR #116625)

[Raphael Moreira Zinsly via llvm-commits]

rzinsly wrote:

@topperc thanks for the review!

This is my first patch and I'm not sure who should be reviewing and pushing this, @asb perhaps?

https://github.com/llvm/llvm-project/pull/116625