r300: force rc_variable_change_dst to always reuse dst

Issue: currently compiler doesn't allow to compile shader if there's uninitialized read. (which is valid behavior) I tried simplest approach by removing and checking if this helps. 😄

This change fixes Rewrite of inst X failed Can't allocate source for Inst X src_type=X new_index=X new_mask=X errors without introducing regression (at least to my testing on r500). (Used draft for now, because I could skip some.)

Btw this helps a lot with grouping instructions into pairs.