gallium: reorganize fragment shader execution, unbreak sse

gallium: reorganize fragment shader execution, unbreak sse

This is probably going to get further reworked in the near future.

Right now there's a new interface wrapped around each shader execution
mode - exec, sse2, llvm.  The llvm code was disabled already and has
just been moved as-is to a new file, whereas the sse2 and exec code is
actually enabled.

The way the interfaces has turned out suggests to me that the correct
approach is to actually have each shader include a pointer to a quad stage
which will do a better job of encapsulating the execution environment than
what I have here -- that's a second step however.