[SeparateConstOffsetFromGEP] Allow SeparateConstOffsetFromGEP pass to lower GEPs.
If LowerGEP is enabled, it can lower a GEP with multiple indices into GEPs with a single index
or arithmetic operations. Lowering GEPs can always extract structure indices. Lowering GEPs can
also give use more optimization opportunities. It can benefit passes like CSE, LICM and CGP.

Reviewed in http://reviews.llvm.org/D5864
Revert "[ELF] Create input and output section names"

This reverts commit r222311.

Reverting because of failure in the darwin bot.
Revert "[ELF] Rename MergedSection to OutputSection."

This reverts commit r222310.

Not sure which commit is the cause of the failure on the darwin bot. Will need
to revert my changes and commit one change at a time.