[TIRx] Replace vars in buffer strides and elem_offset

[guan404ming]

Why

BufferReplacer passed strides and elem_offset through unchanged, so vars in them were never substituted

How

• Visit and rewrite each strides expr and elem_offset when rebuilding the buffer.
• Add test covering the fix.

[gemini-code-assist]

Code Review

This pull request updates the BufferReplacer in python/tvm/tirx/transform/common.py to correctly mutate variables within a buffer's strides and element offset, resolving a previously marked FIXME. It also adds a unit test to verify this behavior. The review feedback points out a potential TypeError if buffer.strides is None and suggests adding a defensive check to handle this case safely.

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