Add missing vtable anchor's.

The following description is from http://reviews.llvm.org/D15481:

ICmpInst, GetElementPtrInst and PHINode have no anchor functions. This causes the vtable and the type info (if RTTI is enabled in user code) to be emitted in multiple translation units.

Before 3.7, the destructors were the key functions for these nodes, but they have been removed.

There have been discussions about this here: http://lists.llvm.org/pipermail/llvm-dev/2015-August/089010.html and here: http://lists.llvm.org/pipermail/llvm-dev/2015-December/092921.html.

Patch by Visoiu Mistrih Francis

llvm-svn: 255538