Another instruction might write to the second source, and then an
INSTR_INVALID_ENC fault will be raised because the tuple will write to
and read from the register at the same time.
Fixes:
795638767d1 ("pan/bi: Use fused dual source blending")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15250>
if (now->add) {
bi_foreach_src(now->add, src) {
+ /* This is not a real source, we shouldn't assign a
+ * slot for it.
+ */
+ if (now->add->op == BI_OPCODE_BLEND && src == 4)
+ continue;
+
if (!(src == 0 && read_dreg))
bi_assign_slot_read(&now->regs, (now->add)->src[src]);
}