Halffloats have a limited range.  It shouldnt normally come up, but if the scale value passed in produces a small value, the half floats will be denormals, which are slow and/or flust to zero.  This test ensures they behave the same in C and SIMD and tests the performance of denormals.

TEST=TestHalfFloatPlane_denormal
BUG=libyuv:560
R=hubbe@chromium.org

Review URL: https://codereview.chromium.org/2424233004 .