intel/fs/ra: Spill without destroying the interference graph

Instead of re-building the interference graph every time we spill, we
modify it in place so we can avoid recalculating liveness and the whole
O(n^2) interference graph building process.  We make a simplifying
assumption in order to do so which is that all spill/fill temporary
registers live for the entire duration of the instruction around which
we're spilling.  This isn't quite true because a spill into the source
of an instruction doesn't need to interfere with its destination, for
instance.  Not re-calculating liveness also means that we aren't
adjusting spill costs based on the new liveness.  The combination of
these things results in a bit of churn in spilling.  It takes a large
cut out of the run-time of shader-db on my laptop.

Shader-db results on Kaby Lake:

    total instructions in shared programs: 15311224 -> 15311360 (<.01%)
    instructions in affected programs: 77027 -> 77163 (0.18%)
    helped: 11
    HURT: 18

    total cycles in shared programs: 355544739 -> 355830749 (0.08%)
    cycles in affected programs: 203273745 -> 203559755 (0.14%)
    helped: 234
    HURT: 190

    total spills in shared programs: 12049 -> 12042 (-0.06%)
    spills in affected programs: 2465 -> 2458 (-0.28%)
    helped: 9
    HURT: 16

    total fills in shared programs: 25112 -> 25165 (0.21%)
    fills in affected programs: 6819 -> 6872 (0.78%)
    helped: 11
    HURT: 16

    Total CPU time (seconds): 2469.68 -> 2360.22 (-4.43%)

Reviewed-by: Kenneth Graunke <[email protected]>

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