[TargetLowering] Simplify (ctpop x) == 1

Reviewers: craig.topper, spatel, RKSimon, bkramer

Reviewed By: spatel

Subscribers: javed.absar, lebedev.ri, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63004

llvm-svn: 362912