[INTERPRETER] Fix lower bound check for block pointers

[Jokeren]

We forgot to check offset >= 0 previously.

Now that it should match the semantic in the GPU backend

https://github.com/triton-lang/triton/blob/7bce3613755e26953518962d02315dfd343dc50c/lib/Dialect/Triton/Transforms/RewriteTensorPointer.cpp#L136