[mlir] Fix warnings

This patch fixes:

  mlir/lib/ExecutionEngine/SparseTensorRuntime.cpp:296:31: error:
  comparison of integers of different signs: 'int64_t' (aka 'long')
  and 'const uint64_t' (aka 'const unsigned long')
  [-Werror,-Wsign-compare]

  mlir/lib/ExecutionEngine/SparseTensorRuntime.cpp:297:67: error:
  comparison of integers of different signs: 'int64_t' (aka 'long')
  and 'const uint64_t' (aka 'const unsigned long')
  [-Werror,-Wsign-compare]

  mlir/lib/ExecutionEngine/SparseTensorRuntime.cpp:298:31: error:
 comparison of integers of different signs: 'int64_t' (aka 'long') and
 'const uint64_t' (aka 'const unsigned long') [-Werror,-Wsign-compare]

  mlir/lib/ExecutionEngine/SparseTensorRuntime.cpp:479:30: error:
  comparison of integers of different signs: 'int64_t' (aka 'long')
  and 'const uint64_t' (aka 'const unsigned long')
  [-Werror,-Wsign-compare]