i965/fs/nir: Minor refactor of store_output

i965/fs/nir: Minor refactor of store_output

Stop retyping the output of shuffle_64bit_data_for_32bit_write.  It's
always BRW_REGISTER_TYPE_D which is perfectly fine for writing out.
Also, when we change get_nir_src to return something with a 64-bit type
for 64-bit values, the retyping will not be at all what we want.  Also,
retyping the output based on src.type before we whack it back to 32 bits
is a problem because the output is always 32 bits.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>