AMDGPU: Rename gfx9 version of v_add_i32/v_sub_i32

The carry-out opcode is renamed, so eliminate the deceptive _gfx9,
which looked like the encoded instruction. The real encoded version
was named _gfx9_gfx9.

Move it into the VI encoding namespace. The gfx9 namespace is just to
deal with the renamed instructions that reinterpret the opcode. When
codegened, it would fail to find the real instruction since it wasn't
in the right namespace.