PhiNode, SwitchInst, LandingPad and IndirectBr all had virtually identical
logic for growing the hung off uses.
Move it to User so that they can all call a single shared implementation.
Their destructors were all empty after this change and were deleted. They all
have virtual clone_impl methods which can be used as vtable anchors.
Reviewed by Duncan Exon Smith.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239492 91177308-0d34-0410-b5e6-96231b3b80d8