tgsi: add support for flt64 constants

tgsi: add support for flt64 constants

These act like flt32 except they take up two slots, and you
can only add 2 x flt64 constants in one slot.

The main reason they are different is we don't want to match half a flt64
constants against a flt32 constant in the matching code, we need to make
sure we treat both parts of the flt64 as an single structure.

Cleaned up printing/parsing by Ilia Mirkin <imirkin@alum.mit.edu>

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Dave Airlie <airlied@redhat.com>