intel/fs: Use nir_lower_interpolation on gen11+

On gen11, the removed the PLN instruction so we have to emit a pile of
MAD to emulate it.  We may as well do that in NIR so we can optimize and
later schedule it.

Shader-db results on Ice Lake:

    total instructions in shared programs: 17145644 -> 16556440 (-3.44%)
    instructions in affected programs: 11507454 -> 10918250 (-5.12%)
    helped: 35763
    HURT: 42085
    helped stats (abs) min: 1 max: 140 x̄: 19.09 x̃: 18
    helped stats (rel) min: 0.04% max: 37.93% x̄: 15.40% x̃: 14.49%
    HURT stats (abs)   min: 1 max: 248 x̄: 2.22 x̃: 2
    HURT stats (rel)   min: 0.05% max: 50.00% x̄: 5.00% x̃: 2.47%
    95% mean confidence interval for instructions value: -7.67 -7.47
    95% mean confidence interval for instructions %-change: -4.46% -4.29%
    Instructions are helped.

    total loops in shared programs: 4370 -> 4370 (0.00%)
    loops in affected programs: 0 -> 0
    helped: 0
    HURT: 0

    total cycles in shared programs: 360624645 -> 368220857 (2.11%)
    cycles in affected programs: 269631244 -> 277227456 (2.82%)
    helped: 15583
    HURT: 65874
    helped stats (abs) min: 1 max: 28561 x̄: 78.45 x̃: 32
    helped stats (rel) min: <.01% max: 67.81% x̄: 5.38% x̃: 2.44%
    HURT stats (abs)   min: 1 max: 238638 x̄: 133.87 x̃: 20
    HURT stats (rel)   min: <.01% max: 306.25% x̄: 5.81% x̃: 3.97%
    95% mean confidence interval for cycles value: 67.42 119.09
    95% mean confidence interval for cycles %-change: 3.61% 3.73%
    Cycles are HURT.

    total spills in shared programs: 8943 -> 8981 (0.42%)
    spills in affected programs: 1925 -> 1963 (1.97%)
    helped: 44
    HURT: 14

    total fills in shared programs: 21815 -> 21925 (0.50%)
    fills in affected programs: 3511 -> 3621 (3.13%)
    helped: 41
    HURT: 18

    LOST:   70
    GAINED: 14

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/intel/compiler/brw_fs.h

@@ -163,7 +163,6 @@ public:
    bool lower_pack();
    bool lower_regioning();
    bool lower_logical_sends();
-   bool lower_linterp();
    bool lower_integer_multiplication();
    bool lower_minmax();
    bool lower_simd_width();