compiler: move generic cmm optimization logic in NCG to a standalone module

This commit moves GHC.CmmToAsm.cmmToCmm to a standalone module,
GHC.Cmm.GenericOpt. The main motivation is enabling this logic to be
run in the wasm backend NCG code, which is defined in other modules
that's imported by GHC.CmmToAsm, causing a cyclic dependency issue.