[mlir][bytecode] Fix crash when reading DenseIntOrFPElementsAttr with unsupported element type (#184773)
When a bytecode type callback substitutes a type that does not implement
DenseElementTypeInterface (e.g., \!test.i32 replacing i32), the bytecode
reader attempted to reconstruct a DenseIntOrFPElementsAttr with that
type. This unconditionally called getDenseElementBitWidth() which hit an
llvm_unreachable on unsupported types.
Fix this by validating the element type implements
DenseElementTypeInterface in readDenseIntOrFPElementsAttr before
proceeding. If the check fails, a proper diagnostic is emitted and
reading fails gracefully instead of crashing.
Fixes #128317