Cleanup/optimizations of the low-bitdepth inverse TX path for AV1 TX
broke Daala TX in several places; this patch cleans up the cleanup.

Tested against the New Daala TX code that unified LBD/HBD, restores
bit-identical TX behavior.

monty-daalaTX-invzerotest-LBD-s1-2@2017-11-10T08:46:01.822Z ->
  monty-daalaTX-invzerotest-test-s1@2017-11-09T05:09:05.483Z

  PSNR | PSNR Cb | PSNR Cr | PSNR HVS |   SSIM | MS SSIM | CIEDE 2000
0.0000 |  0.0000 |  0.0000 |   0.0000 | 0.0000 |  0.0000 |     0.0000

Change-Id: I58e4de4c71ec5251138ff7816f77777db6f869a3