AVX: Move the ZEXT/ANYEXT DAGCo optimizations to the lowering of these...

AVX: Move the ZEXT/ANYEXT DAGCo optimizations to the lowering of these optimizations. The old test cases still cover all of these lowering/optimizations. The single change that we have is that now anyext does not need to zero a register, because it does not use the exact code path as the zero_extend.

llvm-svn: 171178