[hot-cold-split] Only perform splitting in ThinLTO backend post-link

Summary:
Fix the new PM to only perform hot cold splitting once during ThinLTO,
by skipping it in the pre-link phase.

This was already fixed in the old PM by the move of the hot cold split
pass later (after the early return when PrepareForThinLTO) by r344869.

Reviewers: vsk, sebpop, hiraditya

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

Differential Revision: https://reviews.llvm.org/D53611

llvm-svn: 345096