Support the global instruction selection of add i32, i32.
This patch is minimal support. And this patch referenced ARM and AArch64's GlobalISel implementation.
I use the tablegen to generate the CCAssignFn. And the generated function just support the current requirement of
select add i32, i32. The reason for using the generated function is the globalisel infrastructure handleAssignments function
needs the CCAssignFn as one of the parameters. However, The RISCV backend handwriting the CC_RISCV function to deal
with calling conventions. But this function was not matched the CCAssignFn format. So I have to use the tablegen to generate the CCAssignFn.