[LVI] Use a SmallDenseMap instead of std::map for ValueCacheEntryTy

[LVI] Use a SmallDenseMap instead of std::map for ValueCacheEntryTy

Historically there seems to be some resistance regarding the change to DenseMap
(r147980). However, I couldn't find cases of iterator invalidation for
ValueCacheEntryTy, but only for ValueCache, which I left untouched.

This reduces 20s on an internal testcase. Follow up from r245309.

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

rdar://problem/21320066

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245314 91177308-0d34-0410-b5e6-96231b3b80d8