Fix example LVN pass's copy propagation on block inputs

[sampsyo]

In the examples, lvn.py -p currently fails for this example:

@main {
  a: int = const 42;
.lbl:
  b: int = id a;
  a: int = const 5;
  print b;
}

The optimization incorrectly changes the last line to print a.

The problem is the interaction between copy (id) propagation and basic block inputs (i.e., the fact that a is read before it is written in the second block). One annoying but cleanish solution would be to copy a to a fresh variable right as the block begins because it is read before it is written. Another, much less clean solution would be to detect the first overwrite of a and do the renaming at that point, or to somehow avoid doing copy propagation for the id because the value will be clobbered later.