(1) Partial code generation for internal nodes. Extract val and call

instructions being generated successfully.
(2) Updated DFNode class and BuildDFG pass to make all nodes have struct as
return type.
M    llvm/include/llvm/BuildDFG/BuildDFG.h
M    llvm/include/llvm/IR/DFGraph.h
M    llvm/include/llvm/IR/IntrinsicsVISC.td
M    llvm/lib/Transforms/BuildDFG/BuildDFG.cpp
M    llvm/lib/Transforms/DFG2LLVM_X86/DFG2LLVM_X86.cpp