[DirectX backend] flatten multi-dimensional array into a one-dimensional array.

[python3kgae]

DXIL requires array to be 1D only in https://github.com/Microsoft/DirectXShaderCompiler/blob/main/docs/DXIL.rst#arrays

Need a pass in DirectX backend to flatten multi-dimensional array into a one-dimensional array.

Not limited to group shared. static global variable or local variable need to be flattened as well.

groupshared float a[10][16];
static float b[16][4];

Buffer<float> B;
float foo(int x, int y) {
  float l[16][4];
  for (int i=0;i<16;++i)
     for (int j=0;j<4;++j)
        l[i][j] = B[i] + B[j];
  return l[x][y];
}

The groupshared variable ‘a’, static global variable ‘b’, and local variable ‘l’ all need to be flattened into a one-dimensional array.

AC:

• A new IR path is created that transforms arrays into 1D arrays
  • Collect all multi-dimensional array variable as candidates.
  • Create one-dimensional array to replace the original array.
  • Update debug info.
• Tests are created and are passing