[DAG] Cleanup unused node in SimplifySelectCC.

Delete temporarily constructed node uses for analysis after it's use,
holding onto original input nodes. Ideally this would be rewritten
without making nodes, but this appears relatively complex.

Reviewers: spatel, RKSimon, craig.topper

Subscribers: jdoerfert, hiraditya, deadalnix, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57921

llvm-svn: 356382