This patch eliminates the copying of data when using FLIPADST forward
transforms, by incorporating the necessary data flipping into the
load_buffer_* functions of the SSE2 optimized forward transforms. The
load_buffer_* functions are normally inlined, so the overhead of copying
the data is removed and the overhead of flipping is minimized. Left to
right flipping is still not free, as the columns need to be shuffled in
registers.

To preserve identity between the C and SSE2 implementations, the
appropriate C implementations now also do the data flipping as part of
the transform, rather than relying on the caller for flipping the input.

Overall speedup is about 1.5-2% in encode on my tests. Note that these
are only the forward transforms. Inverse transforms to come in a later
patch.

There are also a few code hygiene changes:
- Fixed some indents of switch statements.
- DCT_DCT transform now always use vp10_fht* functions, which dispatch
  to vpx_fdct* for DCT_DCT (some of them used to call vpx_fdct*
  directly, some of them used to call vp10_fht*).

Change-Id: I93439257dc5cd104ac6129cfed45af142fb64574