nv50/ir: only unspill once ahead of a group of instructions

nv50/ir: only unspill once ahead of a group of instructions

We already semi-did this but the list of uses as unsorted, so it was
unreliable. Sort the uses by bb and serial, and don't unspill for each
instruction in a sequence. (And also don't unspill multiple times for a
single instruction that uses the value in question multiple times.)

This causes a minor reduction in generated instructions for shader-db
(as few programs spill) but more importantly it brings determinism to
each run's output.

On SM10:

total instructions in shared programs : 6387945 -> 6379359 (-0.13%)
total gprs used in shared programs    : 728544 -> 728544 (0.00%)
total local used in shared programs   : 9904 -> 9904 (0.00%)

                local        gpr       inst      bytes
    helped           0           0         322         322
      hurt           0           0           0           0

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>