[NFC][IR] Update StructuralHash comment

This patch updates a comment at the top of the structural hash
implementation that was made invalid by
64da0be. The class is now used for more
than just validating pass return status and is now the sole
implementation rather than being copied from somewhere.

Author: boomanaiden154

llvm/lib/IR/StructuralHash.cpp

@@ -20,8 +20,8 @@ using namespace llvm;
 namespace {
 
 // Basic hashing mechanism to detect structural change to the IR, used to verify
-// pass return status consistency with actual change. Loosely copied from
-// llvm/lib/Transforms/Utils/FunctionComparator.cpp
+// pass return status consistency with actual change. In addition to being used
+// by the MergeFunctions pass.
 
 class StructuralHashImpl {
   uint64_t Hash;