- av1_fht32x32 AVX2 function level time reduction ~89% compared to C.

- av1_fht32x32_avx2() on DCT_DCT improves 42.62% over aom_fdct32x32_avx2()
  But function replacement must go with the corresponding inverse txfm.

- No obvious user level time reduction due to 32x32 TX_TYPE selection.

- Zero high 128b YMM to avoid AVX-SSE transition penalties
  (fix 16x16 case).

- Added 32x32 AVX2 unit tests to verify bitexact.

- AVX2 optimization summary:
  On CPU i7-6700, based on 16x16/32x32 fwd txfm optimization results:
  C to AVX2: function level time reduction, ~86-89%.
  SSE2 to AVX2: function level time reduction, ~51%.

Change-Id: Idd0cd8bf066a61c7117140ef15ab6c1f8eb4b036