[mlir][llvm] Fix negative GEP crash in type consistency (#74859)

Fixes https://github.com/llvm/llvm-project/issues/74453

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The `gepToByteOffset` was implicitly casting an signed integer to an
unsigned integer even though negative dimensions are valid for
`llvm.getelementptr`.

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Co-authored-by: Tobias Gysi <tobias.gysi@nextsilicon.com>