ValueMap: Use DenseMap's find_as mechanism to reduce use list churn.

Otherwise just looking up a value in the map requires creating a VH, adding it to the use lists and destroying it again.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157124 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed