vkd3d-shader/hlsl: Fold basic idempotencies.

While not strictly necessary, this pass allows to remove some unnecessary instructions.

For instance:

float4 a;

float4 main() : sv_target
{
    float4 zero = {0, 0, 0, 0};
    float4 one = {1, 1, 1, 1};

    return a + zero + a * one + zero * one;
}

Compiles to:

ps_4_0
dcl_constantbuffer cb0[1], immediateIndexed
dcl_output o0.xyzw
dcl_temps 1
mov r0.xyzw, cb0[0].xyzw
add r0.xyzw, r0.xyzw, r0.xyzw
mov o0.xyzw, r0.xyzw
ret

Instead of

ps_4_0
dcl_constantbuffer cb0[1], immediateIndexed
dcl_output o0.xyzw
dcl_temps 2
mov r0.xyzw, cb0[0].xyzw
add r1.xyzw, r0.xyzw, l(0.000000, 0.000000, 0.000000, 0.000000)
mul r0.xyzw, r0.xyzw, l(1.000000, 1.000000, 1.000000, 1.000000)
add r0.xyzw, r1.xyzw, r0.xyzw
add r0.xyzw, r0.xyzw, l(0.000000, 0.000000, 0.000000, 0.000000)
mov o0.xyzw, r0.xyzw
ret