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Cycles: disable fast math flags, only use a subset.

Empty BVH nodes are set to NaN which must be preserved all the way to the
tnear <= tfar test which can then give false for empty nodes. This needs
strict semantices and careful argument ordering for min() and max(), so
the second argument is used if either of the arguments is NaN.

Fixes T52635: crash in BVH traversal with SSE4.1.

Differential Revision: https://developer.blender.org/D2828