SelectionDAG: when constructing VZEXT_LOAD from other loads, make sure its

SelectionDAG: when constructing VZEXT_LOAD from other loads, make sure its
output chain is correctly setup.

As an example, if the original load must happen before later stores, we need
to make sure the constructed VZEXT_LOAD is constrained to be before the stores.

rdar://11457792

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163036 91177308-0d34-0410-b5e6-96231b3b80d8