[LoopSimplifyCFG] Re-enable const branch folding by default

Known underlying bugs have been fixed, intensive fuzz testing did not
find any new problems. Re-enabling by default. Feel free to revert if
it causes any functional failures.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353911 91177308-0d34-0410-b5e6-96231b3b80d8