AstGen: avoid generating unreachable defer expressions

[andrewrk]

test {
    blk: {
        defer a();
        break :blk;
    }
}

%15 = extended(struct_decl(parent, Auto, {
  test line(0) hash(aa57e7731b05b3a86ade2d43f978df23): %0 = block_inline({
    %13 = extended(func(test, [], @Ref.void_type, inferror, {
      %1 = dbg_stmt(1, 4)
      %2 = block({
        %3 = dbg_stmt(3, 8)
        %4 = dbg_stmt(2, 14)
        %5 = decl_val("a") token_offset:3:15
        %6 = call_chkused(%5, []) node_offset:3:16
        %7 = break(%2, @Ref.void_value)
        %8 = dbg_stmt(2, 14)
        %9 = decl_val("a") token_offset:3:15
        %10 = call_chkused(%9, []) node_offset:3:16
      }) node_offset:2:10
      %11 = ensure_result_used(%2) node_offset:2:10
      %12 = ret_coerce(@Ref.void_value) token_offset:6:1
    }) (lbrace=0:5,rbrace=5:0) node_offset:1:1
    %14 = break_inline(%0, %13)
  }) node_offset:1:1
}, {}, {})

Here you can see there is an unreachable call to a(). The defer expression should not be generated at the block exit, since the block result was unreachable.