[DAGCombiner] refactor function that hoists bitwise logic; NFCI

  
Added FIXME and TODO comments for lack of safety checks.
This function is a suspect in out-of-memory errors as discussed in
the follow-up thread to r347917:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181203/608593.html

llvm-svn: 348501