Switch this code to use hash_combine_range rather than incremental calls

Switch this code to use hash_combine_range rather than incremental calls
to hash_combine. One of the interfaces could already do this, and the
other can just use a small buffer. This is a much more efficient way to
use the hash_combine interface, although I don't have any particular
benchmark where this code was hot, so I can't measure much of an impact.
It at least doesn't slow anything down.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152200 91177308-0d34-0410-b5e6-96231b3b80d8