[llvm] [RISCV] add load/store misched/PostRA cluster options (PR #149409)

[Min-Yih Hsu via llvm-commits]

https://github.com/mshockwave commented:

I'm generally in favor of removing the existing two options (i.e. `riscv-misched-load-store-clustering` and `riscv-postmisched-load-store-clustering`). This is not a Clang flag and LLVM flags change all the time.

Non blocking, but an alternative solution to avoid having two flags could be making `-riscv-misched-load-store-clustering` (and its post-RA counterpart) to take a list of "load" or "store" instead to enable either or both clusterings. 

https://github.com/llvm/llvm-project/pull/149409