gallium: add opcode and types for 64-bit integers. (v3)

gallium: add opcode and types for 64-bit integers. (v3)

This just adds the basic support for 64-bit opcodes,
and the new types.

v2: add conversion opcodes.
add documentation.
v3:
- make docs more consistent
- change TGSI_OPCODE_I2U64 to TGSI_OPCODE_U2I64

Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v2)
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>