[mlir] Update Values to use new casting infra

[mlir] Update Values to use new casting infra

This allows for using the llvm namespace cast methods instead of the ones on the Value class. The Value class method are kept for now, but we'll want to remove these eventually (with a really long lead time).

Related change: https://reviews.llvm.org/D134327

Differential Revision: https://reviews.llvm.org/D135870