IRBuilder: Downgrade InsertPointGuard's instruction pointer to a raw pointer.

Sadly this loses the checking from AssertingVH, but apparently storing the
end() of a BasicBlock into an AssertingVH has bad consequences as it's not
really an instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192209 91177308-0d34-0410-b5e6-96231b3b80d8