AstGen: fix loop result locations

The main problem was that the loop body was treated as an expression
that was one of the peer result values of a loop, when in reality the
loop body is noreturn and only the `break` operands are the result
values of loops.

This was solved by introducing an override that prevents rvalue() from
emitting a store to result location instruction for loop bodies.

An orthogonal change also included in this commit is switching
`elem_val` index expressions to using `coerced_ty` and doing the
coercion to `usize` inside `Sema`, resulting in smaller ZIR (since the
cast becomes implied).

I also changed the break operand expression to use `reachableExpr`,
introducing a new compile error for double break.

This makes a few more behavior tests pass for `while` and `for` loops.

test/behavior.zig

@@ -145,7 +145,6 @@ test {
                 _ = @import("behavior/bugs/7027.zig");
                 _ = @import("behavior/bugs/7047.zig");
                 _ = @import("behavior/bugs/9584.zig");
-                _ = @import("behavior/bugs/9967.zig");
                 _ = @import("behavior/bugs/10147.zig");
                 _ = @import("behavior/byteswap.zig");
                 _ = @import("behavior/call_stage1.zig");