intel/fs: nir_op_extract_i8 extracts a byte, not a word

Fixes: 6ac2d169 ("i965/fs: Fix extract_i8/u8 to a 64-bit destination")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 4aaf139e)
[Emil: resolve trivial conflicts]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

Conflicts:
	src/intel/compiler/brw_fs_nir.cpp