修复以支持包含布尔字段的 [GCOptimize] 结构体

[svermeulen]

没有这个改动，任何带有 [GCOptimize] 标记且包含布尔字段的结构体都会导致内存分配，因为 Generator.SizeOf 将返回 -1。

[tencent-adm]

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