[X86] In X86DAGToDAGISel::PreprocessISelDAG, make sure we don't access N after we delete it.

If we turn X86ISD::AND into ISD::AND, we delete N. But we were continuing onto the next block of code even though N no longer existed.

Just happened to notice it. I assume asan didn't notice it because we explicitly unpoison deleted nodes and give them a DELETE_NODE opcode.

llvm-svn: 335787