stage2: rework astgen result locations

Motivating test case:

```zig
export fn _start() noreturn {
    var x: u64 = 1;
    var y: u32 = 2;
    var thing: u32 = 1;
    const result = if (thing == 1) x else y;
    exit();
}
```

The main idea here is for astgen to output ideal ZIR depending on
whether or not the sub-expressions of a block consume the result
location. Here, neither `x` nor `y` consume the result location of the
conditional expression block, and so the ZIR should communicate the
result of the condbr using break instructions, not with the result
location pointer.

With this commit, this is accomplished:

```
  %22 = alloc_inferred()
  %23 = block({
    %24 = const(TypedValue{ .ty = type, .val = bool})
    %25 = deref(%18)
    %26 = const(TypedValue{ .ty = comptime_int, .val = 1})
    %27 = cmp_eq(%25, %26)
    %28 = as(%24, %27)
    %29 = condbr(%28, {
      %30 = deref(%4)
      < there is no longer a store instruction here >
      %31 = break("label_23", %30)
    }, {
      %32 = deref(%11)
      < there is no longer a store instruction here >
      %33 = break("label_23", %32)
    })
  })
  %34 = store_to_inferred_ptr(%22, %23) <-- the store is only here
  %35 = resolve_inferred_alloc(%22)
```

However if the result location gets consumed, the break instructions
change to break_void, and the result value is communicated only by the
stores, not by the break instructions.

Implementation:

 * The GenZIR scope that conditional branches uses now has an optional
   result location pointer field and a count of how many times the
   result location ended up being an rvalue (not consumed).
 * When rvalue() is called on a result location for a block, it
   increments this counter. After generating the branches of a block,
   astgen for the conditional branch checks this count and if it is 2
   then the store_to_block_ptr instructions are elided and it calls
   rvalue() using the block result (which will account for peer type
   resolution on the break operands).

astgen has many functions disabled until they can be reworked with these
new semantics. That will be done before merging the branch.

There are some new rules for astgen to follow regarding result locations
and what you are allowed/required to do depending on which one is passed
to expr(). See the updated doc comments of ResultLoc for details.

I also changed naming conventions of stuff in this commit, sorry about
that.

0 files changed, 0 insertions, 0 deletions