Introduce MaxUsesToExplore argument to capture tracking

Currently CaptureTracker gives up if it encounters a value with more than 20 
uses. The motivation for this cap is to keep it relatively cheap for 
BasicAliasAnalysis use case, where the results can't be cached. Although, other 
clients of CaptureTracker might be ok with higher cost. This patch introduces an 
argument for PointerMayBeCaptured functions to specify the max number of uses to 
explore. The motivation for this change is a downstream user of CaptureTracker, 
but I believe upstream clients of CaptureTracker might also benefit from more 
fine grained cap.

Reviewed By: hfinkel

Differential Revision: https://reviews.llvm.org/D55042

llvm-svn: 347910