For the downstream RISCV maintenance, it would be easier to inherent
RISCVISelDAGToDAG by including header and only override the method that needs
to be customized for the provider non-standard ISA extension without touching
RISCVISelDAGToDAG.cpp which may cause conflict when upgrading the downstream
LLVM version.
Differential Revision: https://reviews.llvm.org/D77117