[SamplePGO] More pipeline changes when flattened profile used in ThinLTO postlink

Summary:
Follow on to D54819/r351476.

We also don't need to perform extra InstCombine pass when we aren't
loading the sample profile in the ThinLTO backend because we have a
flattened sample profile.

Additionally, for consistency and clarity, when we aren't reloading the
sample profile, perform ICP in the same location as non-sample PGO
backends. To this end I have moved the ICP invocation for non-SamplePGO
ThinLTO down into buildModuleSimplificationPipeline (partly addresses
the FIXME where we were previously setting this up).

Reviewers: wmi

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57705

llvm-svn: 353135