llvmpipe: avoid generating tri_16 for tris which extend past tile bounds

llvmpipe: avoid generating tri_16 for tris which extend past tile bounds

Don't trim triangle bounding box to scissor/draw-region until after
the logic for emitting tri_16.  Don't generate tri_16 commands for
triangles with untrimmed bounding boxes outside the current tile.

This is important as the tri-16 itself can extend past tile bounds and
we don't want to add code to it to check against tile bounds (slow) or
restrict it to locations within a tile (pessimistic).