Do not check deadStripNever twice.

Atoms with deadStripNever attribute has already been added to the
dead strip root set at end of Resolver::doDefinedAtom, so no need
to check it for each atom again.

Differential Revision: http://llvm-reviews.chandlerc.com/D3282

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205575 91177308-0d34-0410-b5e6-96231b3b80d8