Converting IR to selectionDAG

An IR instruction can be represented by an SDAG node. The whole set of instructions thus forms an interconnected directed acyclic graph, with each node corresponding to an IR instruction.

For example, consider the following LLVM IR:

$ cat test.ll
define i32 @test(i32 %a, i32 %b, i32 %c) {
%add = add nsw i32 %a, %b
%div = sdiv i32 %add, %c
ret i32 %div
}

LLVM provides a SelectionDAGBuilder interface to create DAG nodes corresponding to IR instructions. Consider the binary operation:

 %add = add nsw i32 %a, %b

The following function is called when the given IR is encountered:

void SelectionDAGBuilder::visit(unsigned Opcode, const User &I) {
  // Note: this doesn't use InstVisitor, because it has to work with
  // ConstantExpr's in addition to instructions.
  switch (Opcode) {
  default: llvm_unreachable("Unknown instruction type encountered!");
    // Build the switch statement using the Instruction.def file.
#define HANDLE_INST(NUM, OPCODE...