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

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.

This is a reapply of r260458, that was reverted because it was
suspected to be the cause of instability of an internal bot, but
wasn't confirmed.

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

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 262812