i965/vec4: Make sure that register types always match during emit_urb_slot()

i965/vec4: Make sure that register types always match during emit_urb_slot()

Instead of relying on backends (currently vec4_visitor and soon NIR-vec4) to
store registers in output_reg with the correct type, this patch makes sure
that the common code in emit_urb_slot() always emit MOVs from output registers
using the same type on source and destination.

Since the actual type is not important, only that they match, we default to
float.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>