MIR support is totally unusable for AMDGPU without this, since the set
of reserved registers is set from fields here.
Add a clone method to MachineFunctionInfo. This is a subtle variant of
the copy constructor that is required if there are any MIR constructs
that use pointers. Specifically, at minimum fields that reference
MachineBasicBlocks or the MachineFunction need to be adjusted to the
values in the new function.
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
