This is perhaps subtle, but note that this will cause init_sm4_lookup_tables() to map VKD3DSPR_SSA (like VKD3DSPR_UNDEF) to VKD3D_SM4_RT_TEMP, while ideally that lookup would fail.
The lookup table stores pointers, so null is returned for the new register types.
Ah yes, my bad.
I think this MR is ok like this.