Hexagon: Stop adding kill flags on virtual registers (#239040)
Continue purging virtual register uses of kill flags.
HexagonExpandCondsets still uses kill flags internally to decide
predication, so compute them as before but clear them on all touched
registers before the pass returns instead of recomputing them. This
needs a later kill flag management cleanup.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>