Sketch out loop unrolling transformation.

- Implement a full loop unroll for innermost loops.
- Use it to implement a pass that unroll all the innermost loops of all
  mlfunction's in a module. ForStmt's parsed currently have constant trip
  counts (and constant loop bounds).
- Implement StmtVisitor based (Visitor pattern)

Loop IVs aren't currently parsed and represented as SSA values. Replacing uses
of loop IVs in unrolled bodies is thus a TODO. Class comments are sparse at some places - will add them after one round of comments.

A cmd-line flag triggers this for now.

Original:

mlfunc @loops() {
  for x = 1 to 100 step 2 {
    for x = 1 to 4 {
      "Const"(){value: 1} : () -> ()
    }
  }
  return
}

After unrolling:

mlfunc @loops() {
  for x = 1 to 100 step 2 {
    "Const"(){value: 1} : () -> ()
    "Const"(){value: 1} : () -> ()
    "Const"(){value: 1} : () -> ()
    "Const"(){value: 1} : () -> ()
  }
  return
}

PiperOrigin-RevId: 205933235