[mlir] Fix deprecation warnings (NFC)

This patch fixes a couple of warnings by switching to has_value/value:

  mlir/lib/Dialect/Vector/IR/VectorOps.cpp:529:28: error: 'hasValue'
  is deprecated: Use has_value
  instead. [-Werror,-Wdeprecated-declarations]

  mlir/lib/Dialect/Vector/IR/VectorOps.cpp:533:48: error: 'getValue'
  is deprecated: Use value
  instead. [-Werror,-Wdeprecated-declarations]