vkd3d-shader/hlsl: Lower casts from/to ints for SM1.

NOTE: In the end I decided to send the first part of this MR as !616 (merged), already merged.

For temporary registers, SM1-SM3 integer types are internally represented as floating point, so, in order to perform a cast from ints to floats we need a mere MOV.

By the same token, casts from floats to ints can also be implemented with a FLOOR + MOV, where the FLOOR is then lowered by the lower_floor() pass.

For constant integer registers "iN" there is no operation for casting from a floating point register to them. For address registers "aN", and the loop counting register "aL", vertex shaders have the "mova" operation but we haven't used these registers in any way yet.

We probably would want to introduce these as synthetic variables allocated in a special register set. In that case we have to remember to use MOVA instead of MOV in the store operations, but they shouldn't be src or dst of CAST operations.

Regarding constant integer registers, in some shaders, constants are expected to be received formatted as an integer, such as:

int m;
float4 main() : sv_target
{
    float4 res = {0, 0, 0, 0};

    for (int k = 0; k < m; ++k)
        res += k;
    return res;
}

which compiles as:

// Registers:
//
//   Name         Reg   Size
//   ------------ ----- ----
//   m            i0       1
//

ps_3_0
def c0, 0, 1, 0, 0
mov r0, c0.x
mov r1.x, c0.x
rep i0
  add r0, r0, r1.x
  add r1.x, r1.x, c0.y
endrep
mov oC0, r0

but this only happens if the integer constant is used directly in an instruction that needs it, and as I said there is no instruction that allows converting them to a float representation.

Notice how a more complex shader, that performs operations with this integer variable "m":

int m;
float4 main() : sv_target
{
    float4 res = {0, 0, 0, 0};

    for (int k = 0; k < m * m; ++k)
        res += k;
    return res;
}

gives the following output:

// Registers:
//
//   Name         Reg   Size
//   ------------ ----- ----
//   m            c0       1
//

ps_3_0
def c1, 0, 0, 1, 0
defi i0, 255, 0, 0, 0
mul r0.x, c0.x, c0.x
mov r1, c1.y
mov r0.y, c1.y
rep i0
  mov r0.z, r0.x
  break_ge r0.y, r0.z
  add r1, r0.y, r1
  add r0.y, r0.y, c1.z
endrep
mov oC0, r1

Meaning that the uniform "m" is just stored as a floating point in "c0", the constant integer register "i0" is just set to 255 (hoping it is a high enough value) using "defi", and the "break_ge" involving c0 is used to break from the loop.

We could potentially use this approach to implement loops from SM3 without expecting the variables being received as constant integer registers.

According to the D3D documentation, for SM1-SM3 constant integer registers are only used by the 'loop' and 'rep' instructions.