[HotColdSplit] Calculate BFI lazily to reduce compile-time, NFC

The splitting pass does not need BFI unless the Module actually has a profile
summary. Do not calcualte BFI unless the summary is present.

For the sqlite3 amalgamation, this reduces time spent in the splitting pass
from 0.4% of the total to under 0.1%.

llvm-svn: 351894