Revert "[Constant Hoisting] Lazily compute the idom and cache the result."

This code is no longer usefull, because we only compute and use the
IDom once. There is no benefit in caching it anymore.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205498 91177308-0d34-0410-b5e6-96231b3b80d8