D380 caused fft2 regressions
When I got rid of the hand-unboxing in GHC.List, I missed the fact that a
take helper function took an unboxed value and was therefore strict. When I changed that to a regular function, I didn't make it strict, and this caused a regression in fft2 and (I believe) also a test suite perf test. The function is properly strictified in D390.