[DirectX] Implement Shader Flags Analysis for Max64UAVs

[bharadwajy]

Implement analysis to set the module-level flag Max64UAVs. This flag is named m_b64UAVs in dxc

Set if number of UAVs used is > 8 - see here.

This flag is set in the resulting DML Shader compilation.

AC:
Add tests to verify the flag is set.