[DAGCombine] Do not try to deduplicate commutative operations if both operand are the same.

Summary: It is creating useless work as the commuted nodes is the same as the node we are working on in that case.

Reviewers: jyknight, nemanjai, mkuper, spatel, RKSimon, zvi, bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33840

llvm-svn: 310832