When using FLIPADST, the vp10_inv_txfm_add functions used to flip
the destination array, add the result of the inverse transform, to it
and then flip the destination back. This has been replaced by
flipping the result of the inverse transform before adding it to the
destination. Up-Down flipping is done by negating the destination
stride, and staring from the bottom, so it should now be free.
Left-right flipping is done with the usual SSE2 instructions in the
optimized code.

The C functions match the SSE2 functions as expected, so the C functions
now do the flipping as well when required. Adding this cleanly required
some refactoring of the C functions, but there is no measurable
performance impact when ext-tx is not enabled.

Encode speedup with ext-tx enabled is about 3%.

Change-Id: I5b04e5d720f0b9f0d54fd8607a8764f2314c7234