[PM] Refactor the core logic to run EarlyCSE over a function into an

[PM] Refactor the core logic to run EarlyCSE over a function into an
object that manages a single run of this pass.

This was already essentially how it worked. Within the run function, it
would point members at *stack local* allocations that were only live for
a single run. Instead, it seems much cleaner to have a utility object
whose lifetime is clearly bounded by the run of the pass over the
function and can use member variables in a more direct way.

This also makes it easy to plumb the analyses used into it from the pass
and will make it re-usable with the new pass manager.

No functionality changed here, its just a refactoring.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227162 91177308-0d34-0410-b5e6-96231b3b80d8