Refine the PGOOpt and SamplePGOSupport handling.

Summary:
Now that SamplePGOSupport is part of PGOOpt, there are several places that need tweaking:
1. AddDiscriminator pass should *not* be invoked at ThinLTOBackend (as it's already invoked in the PreLink phase)
2. addPGOInstrPasses should only be invoked when either ProfileGenFile or ProfileUseFile is non-empty.
3. SampleProfileLoaderPass should only be invoked when SampleProfileFile is non-empty.
4. PGOIndirectCallPromotion should only be invoked in ProfileUse phase, or in ThinLTOBackend of SamplePGO.

Reviewers: chandlerc, tejohnson, davidxl

Reviewed By: chandlerc

Subscribers: sanjoy, mehdi_amini, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D36040

llvm-svn: 309478