[HotColdSplit] Move splitting after instrumented PGO use

Summary:
Follow up to D57082 which moved splitting earlier in the pipeline, in
order to perform it before inlining. However, it was moved too early,
before the IR is annotated with instrumented PGO data. This caused the
splitting to incorrectly determine cold functions.

Move it to just after PGO annotation (still before inlining), in both
pass managers.

Reviewers: vsk, hiraditya, sebpop

Subscribers: mehdi_amini, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57805

llvm-svn: 353270