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>
34 jobs for !10799 with aco_fix_ra_vector in 1 minute and 19 seconds (queued for 2 seconds)
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