Remove the misguided extension here that reserved two special values in

Remove the misguided extension here that reserved two special values in
the hash_code. I'm not sure what I was thinking here, the use cases for
special values are in the *keys*, not in the hashes of those keys.

We can always resurrect this if needed, or clients can accomplish the
same goal themselves. This makes the general case somewhat faster (~5
cycles faster on my machine) and smaller with less branching.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151865 91177308-0d34-0410-b5e6-96231b3b80d8