DAGCombiner: fix use-counting issue when forming zextload

DAGCombiner was counting all uses of a load node  when considering whether it's
worth combining into a zextload. Really, it wants to ignore the chain and just
count real uses.

rdar://problem/13896307

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185419 91177308-0d34-0410-b5e6-96231b3b80d8