radeonsi: create si_llvm_load_input_gs()

radeonsi: create si_llvm_load_input_gs()

This creates a common function that can be shared by the tgsi
and nir backends.

v2: use LLVMBuildBitCast() directly

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>