[llvm] [RISCV] Remove getPostRAMutations (PR #117527)

[Michael Maitland via llvm-commits]

https://github.com/michaelmaitland approved this pull request.

LGTM.

On another note, have we looked at using the MacroFusionDAGMutation for PostMachineSched?

https://github.com/llvm/llvm-project/pull/117527