intel/fs: Coalesce when the src live range is contained in the dst

intel/fs: Coalesce when the src live range is contained in the dst

Consider the following case:

    // g119-123 are written somewhere above
    mul.sat(16)   g67<1>F    g6.4<0,1,0>F   g125<8,8,1>F
    mul.sat(16)   g69<1>F    g6.5<0,1,0>F   g125<8,8,1>F
    mul.sat(16)   g71<1>F    g6.6<0,1,0>F   g125<8,8,1>F
    mov(16)       g119<1>F   g67<8,8,1>F
    mov(16)       g121<1>F   g69<8,8,1>F
    mov(16)       g123<1>F   g71<8,8,1>F

We should be able to coalesce it into

    mul.sat(16)   g119<1>F   g6.4<0,1,0>F   g125<8,8,1>F
    mul.sat(16)   g121<1>F   g6.5<0,1,0>F   g125<8,8,1>F
    mul.sat(16)   g123<1>F   g6.6<0,1,0>F   g125<8,8,1>F

What's stopping us is an overly conservative check for writes to the two
registers being coalesced.  The check walks over the intersection of
their live ranges and checks for no writes to either one.  However,
because the register which starts the live range (the mul.sat in this
case) is inside that intersection, we flag it as a write in the
intersection and don't coalesce.  However, this case is safe because the
destination register of the copy is never read after the source is
written.

Shader-db changes on ICL:

    total instructions in shared programs: 16043613 -> 16042610 (<.01%)
    instructions in affected programs: 43036 -> 42033 (-2.33%)
    helped: 226
    HURT: 0
    helped stats (abs) min: 1 max: 30 x̄: 4.44 x̃: 4
    helped stats (rel) min: 0.09% max: 26.67% x̄: 4.89% x̃: 3.43%
    95% mean confidence interval for instructions value: -4.86 -4.02
    95% mean confidence interval for instructions %-change: -5.57% -4.22%
    Instructions are helped.

    total cycles in shared programs: 334766372 -> 334710124 (-0.02%)
    cycles in affected programs: 617548 -> 561300 (-9.11%)
    helped: 214
    HURT: 2
    helped stats (abs) min: 15 max: 1512 x̄: 263.21 x̃: 212
    helped stats (rel) min: 0.30% max: 75.36% x̄: 25.30% x̃: 21.58%
    HURT stats (abs)   min: 40 max: 40 x̄: 40.00 x̃: 40
    HURT stats (rel)   min: 0.15% max: 0.15% x̄: 0.15% x̃: 0.15%
    95% mean confidence interval for cycles value: -277.91 -242.90
    95% mean confidence interval for cycles %-change: -27.58% -22.55%
    Cycles are helped.

No spill/fill changes or gained/lost

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4627>