[PGO] Handle cases of failing to split critical edges

Fix PR41279 where critical edges to EHPad are not split.
The fix is to not instrument those critical edges. We used to be able to know
the size of counters right after MST is computed. With this, we have to
pre-collect the instrument BBs to know the size, and then instrument them.

Differential Revision: https://reviews.llvm.org/D62439

llvm-svn: 361882