[RISCV] Store/restore RISCVMachineFunctionInfo into MIR YAML file

RISCVMachineFunctionInfo has some fields like VarArgsFrameIndex and
VarArgsSaveSize are calculated at ISel lowering stage, those info are
not contained in MIR files, that cause test cases rely on those field
can't not reproduce correctly by MIR dump files.

This patch adding the MIR read/write for those fields.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D123178