Cycles: simplify handling of ray differentials

* Store compact ray differentials in ShaderData and compute full differentials
  on demand. This reduces register pressure on the GPU.
* Remove BSDF differential code that was effectively doing nothing as the
  differential orientation was discarded when making it compact.

This gives a 1-5% speedup with RTX A6000 + OptiX in our benchmarks, with the
bigger speedups in simpler scenes.

Renders appear to be identical except for the Both displacement option that
does both displacement and bump.

Differential Revision: https://developer.blender.org/D15677

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