[mlir] Fix printer when it is a DenseElementsAttr of i1

A large DenseElementsAttr of i1could trigger a bug in printer/parser roundtrip.

Ex. A DenseElementsAttr of i1 with 200 elements will print as Hex format of length 400 before the fix. However, when parsing the printed text, an error will be triggered. After fix, the printed length will be 50.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D122925