Revert r1335 and r1336. It was a valiant effort, but on Windows, xmm8-xmm15 are...

Revert r1335 and r1336.  It was a valiant effort, but on Windows, xmm8-xmm15 are non-volatile, and the overhead of pushing them onto the stack at the beginning of each function and popping them at the end was causing worse performance (in the neighborhood of 3-5%) than just using the work areas and limiting the register usage to xmm0-xmm7.  Best to leave the SSE2 code alone.  We can optimize the register usage for AVX2, once that port takes place.

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@1382 632fc199-4ca6-4c93-a231-07263d6284db