[Encoding][DT][NFC] Simplify the codes for EncodingAttr creation.

[hanhanW]

We've developed few builders for the EncodingAttr; the method takes
non-attribute types. We can use the builders to avoid C++ structures to
MLIR attributes conversion.

[bjacob]

This is the only part of this PR that, instead of simplifying code, adds more complex code. Why can't we continue to just pass encoding.getBcastMap() to IREE::Encoding::EncodingAttr::get like in the existing code?

[hanhanW]

Good question. This is because encoding.getBcastMap() returns an attribute (i.e., AffineMapAttr), while the builder requires C++ structured/builtin types (i.e., std::optional<AffineMap>). Instead of having the logic here, I think we can add an std::optional<AffineMap> getBcastMapValue() method to the encoding attribute. What do you think?

[bjacob]

I see. I like the idea, but really don't favor adding extraClassDeclarations. Can this be a plain global C++ function instead (not declared in .td, not member)? Otherwise, feel free to just drop that commit, I don't want to make this PR hard to merge and I liked the original form very much already.

[hanhanW]

I can try to put it to a better state when I start poking the broadcast map things. I don't really know what methods could be useful and we can revisit later (when we have more needs). Revert the commits for now.