[HotColdSplit] Schedule splitting late to fix perf regression

With or without PGO data applied, splitting early in the pipeline
(either before the inliner or shortly after it) regresses performance
across SPEC variants. The cause appears to be that splitting hides
context for subsequent optimizations.

Schedule splitting late again, in effect reversing r352080, which
scheduled the splitting pass early for code size benefits (documented in
https://reviews.llvm.org/D57082).

Differential Revision: https://reviews.llvm.org/D58258

llvm-svn: 354158