[LVI] Use DenseMap::find_as in LazyValueInfo.

Summary:
This lets us avoid creating and destroying a CallbackVH every time we
check the cache.

This is good for a 2% e2e speedup when compiling one of the large Eigen
tests at -O3.

FTR, I tried making the ValueCache hashtable one-level -- i.e., mapping
a pair (Value*, BasicBlock*) to a lattice value, and that didn't seem to
provide any additional improvement.  Saving a word in LVILatticeVal by
merging the Tag and Val fields also didn't yield a speedup.

Reviewers: reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D21951

llvm-svn: 276926