gallivm: add lp_build_emit_fetch_src() helper

gallivm: add lp_build_emit_fetch_src() helper

lp_build_emit_fetch() is useful when the source type can be
infered from the instruction opcode.

However, for bindless samplers/images we can't do that easily
because tgsi_opcode_infer_src_type() returns TGSI_TYPE_FLOAT for
TEX instructions, while we need TGSI_TYPE_UNSIGNED64 if the
resource register is bindless.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>