[mlir][OpFormatGen][NFC] Change Raw{Operands,Types} arrays to objects (#85631)

Tablegen generates uninitialized arrays of size 1 for raw operands and
types. In the current state this causes static analysis warnings about
"uninitialized fixed-size arrays" as their init is separated from their
declaration. Since these are single-entry array, we can just use a plain
variable instead of an array here.

Co-authored-by: Orest Chura <orest.chura@intel.com>
Co-authored-by: Mehdi Amini <joker.eph@gmail.com>