[LICM] *Actually* disable ControlFlowHoisting.

Summary:
The remaining code paths that ControlFlowHoisting introduced that were
not disabled, increased compile time by 3x for some benchmarks.
The time is spent in DominatorTree updates.

Reviewers: john.brawn, mkazantsev

Subscribers: sanjoy, jlebar, llvm-commits

Differential Revision: https://reviews.llvm.org/D55313

llvm-svn: 348345