[LVI] Improve LazyValueInfo compile time performance

Changes in LoopUnroll in the past six months exposed scalability
issues in LazyValueInfo when used from JumpThreading. One internal test
that used to take 20s under -O2 now takes 6min.

This commit change the OverDefinedCache from
DenseSet<std::pair<AssertingVH<BasicBlock>, Value*>> to
DenseMap<AssertingVH<BasicBlock>, SmallPtrSet<Value *, 4>>
and reduces compile time down to 1m40s.

Differential Revision: http://reviews.llvm.org/D11651

rdar://problem/21320066

llvm-svn: 245309