Eliminate another redundant instruction in vp56/8 arithcoder

Necessary because of this GCC bug:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44474
To do this, convert some, but not all (!) of the variables in VP56RangeCoder
into local variables.
If we convert c->high into a local variable, gcc gets the stupids and refuses
to use a conditional move for the unpredictable main branch.

TODO: dispense with this bullshit and write an asm version.

Originally committed as revision 23924 to svn://svn.ffmpeg.org/ffmpeg/trunk