[mlir] Fix -Wsign-compare in ValueBoundsOpInterface.cpp (NFC)

/llvm-project/mlir/lib/Interfaces/ValueBoundsOpInterface.cpp:762:16:
error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
        rhsPos >= cstr.positionToValueDim.size())
        ~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/llvm-project/mlir/lib/Interfaces/ValueBoundsOpInterface.cpp:761:16:
error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'size_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
    if (lhsPos >= cstr.positionToValueDim.size() ||
        ~~~~~~ ^  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.