llvmpipe: optimize case when all four pixels are inside the triangle

llvmpipe: optimize case when all four pixels are inside the triangle

When the incoming c0,c1,c2 values are equal to INT_MIN it means that
all pixels are inside the triangle.  Thus we can skip the detailed
pixel inside/outside triangle tests.  Use the new lp_build_if()/endif()
functions to generate the branching code.

The code is disabled ATM however because it's actually a little slower
than the original code.  A little more tuning may fix that though...