libLTO, llvm-lto, gold: Introduce flag for controlling optimization level.

libLTO, llvm-lto, gold: Introduce flag for controlling optimization level.

This change also introduces a link-time optimization level of 1. This
optimization level runs only the globaldce pass as well as cleanup passes for
passes that run at -O0, specifically simplifycfg which cleans up lowerbitsets.

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150316/266951.html

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232769 91177308-0d34-0410-b5e6-96231b3b80d8