Fix UB in float3_optimization performance test.
Fix UB in float3_optimization performance test.

The rotate_tuple kernel was returning references to stack memory.  nvcc
didn't notice or care, but clang did, and optimized away the whole
function.  I suppose it was equally correct, and it was indeed faster.
:)

Fixes thrust/thrust#769.

egaburov/thrust,sdalton1/thrust,sdalton1/thrust,egaburov/thrust,sdalton1/thrust,egaburov/thrust