aco/ra: prevent underflow register for p_create_vector operands
It could happen that we tested negative out-of-range registers for p_create_vector operands resulting in a crash. Fixes: 8962510e ('aco/ra: Conservatively refactor get_reg_specified to use PhysRegInterval') Closes: #4697 Reviewed-by:Rhys Perry <pendingchaos02@gmail.com> Reviewed-by:
Tony Wasserka <tony.wasserka@gmx.de> Part-of: <mesa/mesa!10799>