Cycles: remove WITH_CYCLES_DEBUG, add WITH_CYCLES_DEBUG_NAN

WITH_CYCLES_DEBUG was used for rendering BVH debugging passes. But since we
mainly use Embree an OptiX now, this information is no longer important.

WITH_CYCLES_DEBUG_NAN will enable additional checks for NaNs and invalid values
in the kernel, for Cycles developers. Previously these asserts where enabled in
all debug builds, but this is too likely to crash Blender in scenes that render
fine regardless of the NaNs. So this is behind a CMake option now.

Fixes T90240