i965/fs: add shuffle_32bit_load_result_to_64bit_data helper

There will be a few places where we need to shuffle the result of a 32-bit
load into valid 64-bit data, so extract this logic into a separate helper
that we can reuse.

v2 (Curro):
- Use subscript() instead of stride()
- Assert on the input types rather than retyping.
- Use offset() instead of horiz_offset(), drop the multiplier definition.
- Don't use  force_writemask_all.
- Mark component_i as const.
- Make the function name lower case.

v3 (Curro):
- Pass src and dst by reference.
- Move to brw_fs_nir.cpp

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