[llvm] [RISCV][WIP] Let RA do the CSR saves. (PR #90819)

[Mikhail Gudim via llvm-commits]

mgudim wrote:

About using the `ReachingDefAnalysis`: I wonder if it would be better to use RDF framework for this. This seems like it is capable to do the same thing, but if we make it work for RISCV, that could be some additional gains. Here I extended `ReachingDefAnalysis` to work with stack slots because it was easier (for me) to understand.


https://github.com/llvm/llvm-project/pull/90819