Change the chain input of nodes that load the address of a function. This change

Change the chain input of nodes that load the address of a function. This change
 enables SelectionDAG::getLoad at MipsISelLowering.cpp:1914 to return a
pre-existing node instead of redundantly create a new node every time it is
called.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133811 91177308-0d34-0410-b5e6-96231b3b80d8