[DirectX] Implement Shader Flags Analysis for LowPrecisionPresent

[bharadwajy]

Set module-level flag LowPrecisionPresent if any of the module functions use an instruction with operands of llvm::Type::TypeID::HalfID or Int16Ty

This flag is named m_bLowPrecisionPresent in dxc sources.

AC:
Add tests to verify the flag is set.