i965/fs: Return a fs_reg from shuffle_64bit_data_for_32bit_write

i965/fs: Return a fs_reg from shuffle_64bit_data_for_32bit_write

All callers of this function allocate a fs_reg expressly to pass into
it.  It's much easier if we just let the helper allocate the register.
While we're here, we switch it to doing the MOVs with an integer type so
that we don't accidentally canonicalize floats on half of a double.

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