[X86] When removing sign extends from gather/scatter indices, make sure we...

[X86] When removing sign extends from gather/scatter indices, make sure we handle UpdateNodeOperands finding an existing node to CSE with.

If this happens the operands aren't updated and the existing node is returned. Make sure we pass this existing node up to the DAG combiner so that a proper replacement happens. Otherwise we get stuck in an infinite loop with an unoptimized node.

llvm-svn: 338090