X86: when constructing VZEXT_LOAD from other loads, makes sure its output

X86: when constructing VZEXT_LOAD from other loads, makes 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://12684358

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167859 91177308-0d34-0410-b5e6-96231b3b80d8