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/while.zig

@@ -236,3 +236,33 @@ test "while on error union with else result follow break prong" {
     } else |_| @as(i32, 2);
     try expect(result == 10);
 }
+
+test "while bool 2 break statements and an else" {
+    const S = struct {
+        fn entry(t: bool, f: bool) !void {
+            var ok = false;
+            ok = while (t) {
+                if (f) break false;
+                if (t) break true;
+            } else false;
+            try expect(ok);
+        }
+    };
+    try S.entry(true, false);
+    comptime try S.entry(true, false);
+}
+
+test "while optional 2 break statements and an else" {
+    const S = struct {
+        fn entry(opt_t: ?bool, f: bool) !void {
+            var ok = false;
+            ok = while (opt_t) |t| {
+                if (f) break false;
+                if (t) break true;
+            } else false;
+            try expect(ok);
+        }
+    };
+    try S.entry(true, false);
+    comptime try S.entry(true, false);
+}