stage2: implement a pass on AIR which does dead store elimination

[FireFox317]

The following function generates invalid TZIR (changing const val to var val fixes it):

fn foo(ok: bool) void {
    const val: i32 = blk: {
        if (!ok) break :blk 10;
        break :blk 20;
    };
    var other: i32 = val;
}

TZIR output (I implemented TZIR printing for blocks etc, PR incoming):

Module.Function(name=foo):
  @1: i32 = 10;
  @2: void = {};
  @4: i32 = 20;
  %0: bool = arg(ok)
  %1: void = dbg_stmt()
  %2: *i32 = alloc()
  %3: i32 = block(
    %4: void = dbg_stmt()
    %5: void = block(
      %6: bool = not(%0)
      %7: noreturn = condbr(%6,
      then:
        %8: void = store(%2, @1)
        %9: noreturn = br(%3, @1)
      else:
        %10: noreturn = br(%5, @2)
      )
    )
    %11: void = dbg_stmt()
    %12: void = store(%2, @4)
    %13: noreturn = br(%3, @4)
  )
  %14: void = dbg_stmt()
  %15: *i32 = alloc()
  %16: void = store(%15, %3)
  %17: noreturn = retvoid()

The problem happens at instruction %16. Here we try to store %3 in %15, however %3 is the block instruction. But as you can see in instruction %8, we store the value in %2. So I think the problem is that instruction %16 should change to:

%16: void = store(%15, %2)

[FireFox317]

I think I know what is going wrong, I'm working on it.

[FireFox317]

The current AIR generation is working fine, however there are dead stores that can be eliminated from the AIR. Basically all the stores to %2 can be eliminated, since there are no loads from that instruction.