i965: Make opt_vector_float() only handle non-type-conversion MOVs.

i965: Make opt_vector_float() only handle non-type-conversion MOVs.

We don't handle this properly - we'd have to perform the type conversion
before trying to convert the value to a VF.

While we could do that, it doesn't seem particularly useful - most
vector loads should be consistently typed (all float or all integer).

As a special case, we do allow type-converting MOVs of integer 0, as
it's represented the same regardless of the type.  I believe this case
does actually come up.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>