gallivm/llvmpipe: prepare support for ARB_gpu_shader_int64.

gallivm/llvmpipe: prepare support for ARB_gpu_shader_int64.

This enables 64-bit integer support in gallivm and
llvmpipe.

v2: add conversion opcodes.
v3:
- PIPE_CAP_INT64 is not there yet
- restrict DIV/MOD defaults to the CPU, as for 32 bits
- TGSI_OPCODE_I2U64 becomes TGSI_OPCODE_U2I64

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>