[DirectX] scalarize vector types

[python3kgae]

DXIL requires Vectors to be scalarized.

AC:

• design doc for scalarize vector types.
  Note that group-shared vectors need to be lowered to arrays to maintain layout consistency and avoid affecting access coalescing.
• Verify that the vector overload of intrinsic functions is functioning correctly.
• Introduce the ScalarizerPass to the DirectX backend pipeline.
  Keep in mind that the ScalarizerPass does not have a wrapper for the legacy pass manager.
• Add a pass to scalarize global variables, including static globals and group-shared variables.
• Tests