swift234

Improve ConstantTracker in PerformanceInliner.

This lets the inliner better check if a closure is passed to the callee.
It fixes a problem that copy forwarding generates a pattern which could not be analyzed by the ConstantTracker:
  <rdar://problem/19426897> [Inliner] Fail to fully optimize RangeAssignment (especially with copy forwarding)

RangeAssignment is now ~6x faster with -O. 
Some other improvements: PrimeNum@O: +50%, ImageProc@Ounchecked: +15%, QuickSort@Ounchecked: +20%
There is one degradation in -O and -Ounchecked, which I still have to check: CommonMarkRender: -15%




Swift SVN r24414