stage2: fix stack overflow in @setEvalBranchQuota test case

Some of the reworkings in this branch put us over the limit, on Linux,
where the kernel disregards the fact that we ask for 16 MiB in the ELF
file. So we ask for more stack space in `main`.

src/Module.zig

@@ -1088,7 +1088,7 @@ pub const Scope = struct {
         /// for the one that will be the same for all Block instances.
         src_decl: *Decl,
         instructions: ArrayListUnmanaged(*ir.Inst),
-        label: ?Label = null,
+        label: ?*Label = null,
         inlining: ?*Inlining,
         is_comptime: bool,