Support the global instruction selection of add i32, i32. This patch is the minimal support. And this patch referenced the implementation of ARM and AArch64's GlobalISel.

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.

Support the global instruction selection of add i32, i32. This patch is the minimal supporminimal 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. And this patch referencedSo I have to use the implementation of ARM and AArch64's GlobalISeltablegen to generate the CCAssignFn.