Revert part of r310296 to make it really NFC for instrumentation PGO.

Summary: Part of r310296 will disable PGOIndirectCallPromotion in ThinLTO backend if PGOOpt is None. However, as PGOOpt is not passed down to ThinLTO backend for instrumentation based PGO, that change would actually disable ICP entirely in ThinLTO backend, making it behave differently in instrumentation PGO mode. This change reverts that change, and only disable ICP there when it is SamplePGO.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: sanjoy, mehdi_amini, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D36566

llvm-svn: 310550