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

The splitting pass does not need (post)domtrees until after it's found a
cold block. Defer domtree calculation until a cold block is found.

For the sqlite3 amalgamation, this reduces time spent in the splitting
pass from 0.8% of the total to 0.4%.

llvm-svn: 351892