gallivm: enable fetch for integer opcodes. (v2)

gallivm: enable fetch for integer opcodes. (v2)

The infers the type of data required using the opcode,
and casts the input to the appropriate type.

So far this only handles non-indirect constant and temporaries.

v2: as per Jose suggestion, fetch immediates via floats

Signed-off-by: Dave Airlie <airlied@redhat.com>