[SelectionDAG] Do not salvage with vector node

rG2eb7cbf987f21 added this code, which results in crash for vector
nodes. This patch solves it by skipping for the vector nodes.

Thanks Steve for helping reducing the test case.

Co-authored-by: Steve Merritt <steve.merritt@intel.com>

Reviewed By: goldstein.w.n

Differential Revision: https://reviews.llvm.org/D152492