[RISCV][NFC] Combine RISCVOptWInstrs::stripWSuffixes and appendWSuffixes into canonicalizeWSuffixes (#149710)
This refactor was suggested in
<https://github.com/llvm/llvm-project/pull/144703>.
I have checked for unexpected changes by comparing builds of
llvm-test-suite with/without this refactor, including with preferWInst
force enabled.