[AliasSetTracker] Degrade AliasSetTracker when may-alias sets get too large.

Repeated inserts into AliasSetTracker have quadratic behavior - inserting a
pointer into AST is linear, since it requires walking over all "may" alias
sets and running an alias check vs. every pointer in the set.

We can avoid this by tracking the total number of pointers in "may" sets,
and when that number exceeds a threshold, declare the tracker "saturated".
This lumps all pointers into a single "may" set that aliases every other
pointer.

(This is a stop-gap solution until we migrate to MemorySSA)

This fixes PR28832.
Differential Revision: https://reviews.llvm.org/D23432

llvm-svn: 279274