[RISCV] Merge Masked and unMasked RVV manual codegen

RVV intrinsic function will generate riscv_vector_builtin_cg.inc for CGBuiltin.cpp to produce the corresponding RVV intrinsic LLVM IR.

In this stage, riscv_vector.td will describe the bunch of manual codegen C++ code to tell CGBuiltin how to handle these instructions.

In this patch, we merge the masked RVV manual codegen and unmasked RVV manual codegen to reduce the number of manual codegen, and make more policy addition easier in the future.

This is a clean-up job that will not affect the RVV intrinsic functionality.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D140361