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/bugs/9967.zig

@@ -1,8 +0,0 @@
-const std = @import("std");
-
-test "nested breaks to same labeled block" {
-    const a = blk: {
-        break :blk break :blk @as(u32, 1);
-    };
-    try std.testing.expectEqual(a, 1);
-}