[mlir] Fix DenseElementsAttr treatment of bool splat of "true"

Boolean splats currently can't roundtrip via the "raw" DenseElementsAttr
API. This is because internally we treat true splats in some cases as "1"(one bit set)
and in other cases as "0xFF"(all bits set). This commit cleans up this handling to
consistently use 0xFF (all bits set) as the value for a splat of true.

Differential Revision: https://reviews.llvm.org/D133743