[DAG] Avoid using deleted node in rebuildSetCC

Summary:
The combine in rebuildSetCC may be combined to another
node leaving our references stale. Keep a handle on
it to avoid stale references.

Fixes PR36602.

Reviewers: dbabokin, RKSimon, eli.friedman, davide

Subscribers: hiraditya, uabelho, JesperAntonsson, qcolombet, llvm-commits

Differential Revision: https://reviews.llvm.org/D46404

llvm-svn: 331985