Implement offline variable substitution in order to reduce memory

Implement offline variable substitution in order to reduce memory
and time usage.
Fixup operator == to make this work, and add a resize method to DenseMap
so we can resize our hashtable once we know how big it should be.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42269 91177308-0d34-0410-b5e6-96231b3b80d8