Refactor string.TryCopyTo

[xtqqczze]

Total bytes of base: 40205636
Total bytes of diff: 40201837
Total bytes of delta: -3799 (-0.01 % of base)
Total relative delta: -5.52
    diff is an improvement.
    relative diff is an improvement.

[xtqqczze]

@MihuBot

[EgorBo]

It's not clear why it's better. In fact, your change assumes that the destination is more likely to be not big enough which is not true. Generally, if you think some C# massage improves codegen, please file an issue against JIT instead

[huoyaoyuan]

Moreover, assembly size difference is not the correct measurement of performance. Extra instructions are sometimes required to fine tune CPU behavior, especially branch prediction. "Optimize for size" and "optimize for speed" are also different configurations for compilers, including RyuJIT and C compilers.

To demonstrate a real performance improvement, please include numbers of performance benchmarks, and/or the detailed ASM code for comparison. Generally it's not the goal to minimize the size number only.