cleanup ConstantInt to use a single DenseMap for uniquing instead of the

cleanup ConstantInt to use a single DenseMap for uniquing instead of the
heavy-weight ValueMap class.  This reduces mem usage bc reading kc++ by 29K,
even though it only creates 2955 constant ints!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34445 91177308-0d34-0410-b5e6-96231b3b80d8