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

This reverts rL341954.

The builder `sanitizer-x86_64-linux-bootstrap-ubsan` has been
failing with timeouts at stage2 clang/ubsan:

[3065/3073] Linking CXX executable bin/lld
command timed out: 1200 seconds without output running python
../sanitizer_buildbot/sanitizers/buildbot_selector.py,
attempting to kill

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342001 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed