i965/fs: add shuffle_64bit_data_for_32bit_write helper

i965/fs: add shuffle_64bit_data_for_32bit_write helper

This does the inverse operation of shuffle_32bit_load_result_to_64bit_data
and we will use it when we need to write 64-bit data in the layout expected
by untyped write messages.

v2 (curro):
- Use subscript() instead of stride()
- Assert on the input types rather than silently retyping.
- Use offset() instead of horiz_offset(), drop the multiplier definition.
- Drop the temporary vgrf and force_writemask_all.
- Make component_i const.
- Move to brw_fs_nir.cpp

v3 (curro):
- Pass dst and src by reference.
- Simplify allocation of tmp register.
- Move to brw_fs_nir.cpp.
- Get rid of the temporary.

v3 (Iago):
- Check that the src and dst regions do not overlap, since that would
  typically be a bug in the caller.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>