Revert "GVN-hoist: enable by default" & "Make GVN Hoisting obey optnone/bisect."

This reverts commits r277685 & r277688. r277685 broke compiler-rt
compilation http://lab.llvm.org:8080/green/job/clang-stage1-configure-RA_build/23335
and r277685 is a followup from it.

llvm-svn: 277690