llvmpipe: Take back AVX2 intrinsic support.

The percentage of AVX2 systems out there is so small that it's hard to
justify hand-coding AVX2 intrinsic code paths at this moment.

(Of course, we can support AVX2 in LLVM IR though, but at the moment
rasterization code does not use LLVM JIT.)
llvmpipe: Tweak IMUL64 definition.

It should generate slightly better code for

  IMUL64(x + y, z + w)

as the cast will be deferred to after the addition.
llvmpipe: Disable SSE rasterization code.

But only the rasterization code.
llvmpipe: Try 32-bits dcdx/dcdy.

Should work in theory, and it doesn't seem to make things any worse in
practice.

However triangle-rasterization-overdraw fails with or without this.