Also it's worth noting, I tried building it under XCode 10.3 again, and this same issue is reported as a warning, not an error. Maybe one of the default build settings became more strict with respect to this?
Out of interest, what happens if you change the code to
y = bt_splat_ps(y, 128);
If it doesn't make any difference I would try this
#define BT_SHUFFLE(x, y, z, w) (((w) << 6 | (z) << 4 | (y) << 2 | (x))&255)
Thought that would change the behaviour.
It looks like the shuffle is creating the value 10880
(0x80 << 6) + (0x80 <<4) + (0x80 << 2) + (0x80) = 0x2000 + 0x800 + 0x200 + 0x80 = 0x2a80 = 10,880