Replace MemoryMarshal.GetReference with GetReference extension and reuse base addresses

[Copilot]

Replace MemoryMarshal.GetReference with GetReference extension and reuse computed base addresses

• Update FastUtils.cs (Xor16 methods) - removed unnecessary local variables
• Update GHashX86.cs - removed unnecessary local variables
• Update Salsa20Crypto.cs - removed unnecessary local variables
• Update Salsa20Utils.cs - kept variables where reused multiple times
• Update ChaCha20Utils.cs - kept variables where reused multiple times, removed all single-use variables including stateRef in IncrementCounter
• Update XtsMode.cs - removed unnecessary local variables
• Update CTR128StreamMode.cs - removed unnecessary local variables
• Update CTR128StreamModeBlock4X86.cs - removed unnecessary local variables
• Update CTR128StreamModeBlock16X86.cs - removed unnecessary local variables
• Update CTR128StreamModeX86.cs - removed unnecessary local variables
• Update CTR128StreamModeBlock8X86.cs - removed unnecessary local variables
• Update CTR128StreamModeBlock8AvxX86.cs - removed unnecessary local variables
• Update SM4Utils.cs - kept variables where reused multiple times
• Build and test changes

Pattern applied:

• Base addresses are computed once and stored in local variables only when reused multiple times with Unsafe.Add
• When base address is used only once, GetReference() is called directly inline without intermediate variable

Original prompt

将 MemoryMarshal.GetReference 全部换成 GetReference 扩展；尽可能复用算出来的基址

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[HMBSbige]

@copilot 无法复用的不要局部变量

[Copilot]

@copilot 无法复用的不要局部变量

Removed unnecessary local variables where base address is only used once. Now variables are only created when the base address is reused multiple times (commit 7d49d82).

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[HMBSbige]

@copilot 这里不用局部变量

[Copilot]

Removed the unnecessary stateRef variable (commit 6dc1736).