v3d: Make sure that a thrsw doesn't split a multop from its umul24.

The thrsw will invalidate rtop, just like accumulators and flags.  Caught
by simulator assertions in CS imulextended/umulextended tests.

Fixes: 90269ba3 ("broadcom/vc5: Use THRSW to enable multi-threaded shaders.")
(cherry picked from commit 3f9bcf91)