[PGOMemOPSize] Preserve the DominatorTree

Summary:
PGOMemOPSize only modifies CFG in a couple of places; thus we can preserve the DominatorTree with little effort.
When optimizing SQLite with -O3, this patch can decrease 3.8% of the numbers of nodes traversed by DFS and 5.7% of the times DominatorTreeBase::recalculation is called.

Reviewers: kuhar, davide, dmgreen

Reviewed By: dmgreen

Subscribers: mzolotukhin, vsk, llvm-commits

Differential Revision: https://reviews.llvm.org/D48914

llvm-svn: 336522