[mlir] Fix warnings

This patch fixes:

  mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp:1348:31: error: comparison
  of integers of different signs: 'size_t' (aka 'unsigned long') and
  'int64_t' (aka 'long') [-Werror,-Wsign-compare]

  mlir/lib/ExecutionEngine/SparseTensor/File.cpp:110:3: error: default
  label in switch which covers all enumeration values
  [-Werror,-Wcovered-switch-default]