Add a new insert_as() method to DenseMap and use it for ConstantUniqueMap

Summary:
Just like the existing find_as() method, the new insert_as() accepts
an extra parameter which is used as a key to find the bucket in the
map.
When creating a Constant, we want to check the map before actually
creating the object. In this case we have to perform two queries to
the map, and this extra parameter can save recomputing the hash value
for the second query.

Reviewers: dexonsmith, chandlerc

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16268

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 260458