[PlaceSafepoints] Reduce dominator tree recalculation

[PlaceSafepoints] Reduce dominator tree recalculation

Reduce recalculation of the dominator tree by identifying all sites that will need a safepoint poll before doing any of the insertion. This allows us to invalidate the dominator info once, rather than once per safepoint poll inserted.

While I'm at it, update findLocationForEntrySafepoint to properly update the dom tree now that the interface has been made easy. When first written, it wasn't per comment in the code.

Differential Revision: http://reviews.llvm.org/D9727

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237220 91177308-0d34-0410-b5e6-96231b3b80d8