Fix a silly restriction on the fast-path for hash_combine_range. This

Fix a silly restriction on the fast-path for hash_combine_range. This
caused several clients to select the slow variation. =[ This is extra
annoying because we don't have any realistic way of testing this -- by
design, these two functions *must* compute the same value.

Found while inspecting the output of some benchmarks I'm working on.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152369 91177308-0d34-0410-b5e6-96231b3b80d8