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

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310832 91177308-0d34-0410-b5e6-96231b3b80d8