Remove another outbreak of customized (and completely broken) hashing.

Remove another outbreak of customized (and completely broken) hashing.
This one is particularly annoying because the hashing algorithm is
highly specialized, with a strange "equivalence" definition that subsets
the fields involved.

Still, this looks at the exact same set of data as the old code, but
without bitwise or-ing over parts of it and other mixing badness. No
functionality changed here. I've left a substantial fixme about the fact
that there is a cleaner and more principled way to do this, but it
requires making the equality definition actual stable for particular
types...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152218 91177308-0d34-0410-b5e6-96231b3b80d8