Revert r340922 "[GVNHoist] Re-enable GVNHoist by default"

Another sanitizer buildbot failed this time at bootstrap when
compiling SemaTemplateInstantiate.cpp with this assertion:
`dominates(MD, U) && "Memory Def does not dominate it's uses"'.

http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux/builds/15047

llvm-svn: 340925