[ELF] Don't special case symbolic relocations with 0 addend to ifunc in writable locations

Currently the following 3 relocation types do not trigger the creation
of a canonical PLT (which changes STT_GNU_IFUNC to STT_FUNC and
redirects all references):

1) GOT-generating (`needsGot`)
2) PLT-generating (`needsPlt`)
3) R_ABS with 0 addend in a writable location. This is used for
  for ifunc function pointers in writable sections such as .data and .toc.

This patch deletes case 3) to simplify the R_*_IRELATIVE generating
logic added in D57371. Other advantages:

* It is guaranteed no more than 1 R_*_IRELATIVE is created for an ifunc.
* PPC64: no need to special case ifunc in toc-indirect to toc-relative relaxation. See D65755

The deleted elf::addIRelativeRelocs demonstrates that one-pass scan
through relocations makes several optimizations difficult. This is
something we can think about in the future.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D65995

llvm-svn: 368661