nir: Only float and double types can be matrices

nir: Only float and double types can be matrices

In 19a541f (nir: Get rid of nir_constant_data) a number of places that
operated on nir_constant::values were mechanically converted to operate
on the whole array without regard for the base type.  Only
GLSL_TYPE_FLOAT and GLSL_TYPE_DOUBLE can be matrices, so only those
types can have data in the non-0 array element.

See also b870394.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Cc: Iago Toral Quiroga <itoral@igalia.com>