Revert "Linker: Drop superseded subprograms"

This reverts commit r224389.  Based on feedback from the bots, the
assertion seems to be going off *more* often, not less (previously I was
just seeing it in an internal bootstrap, now it's happening in public
builds too).

http://lab.llvm.org:8080/green/job/clang-stage2-configure-Rlto_build/936/
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap/builds/5325

Reverting in order to investigate.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224416 91177308-0d34-0410-b5e6-96231b3b80d8
Strength reduce intrinsics with overflow into regular arithmetic operations if possible.

Some intrinsics, like s/uadd.with.overflow and umul.with.overflow, are already strength reduced.
This change adds other arithmetic intrinsics: s/usub.with.overflow, smul.with.overflow.
It completes the work on PR20194.




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224417 91177308-0d34-0410-b5e6-96231b3b80d8