[flang] Hanlde disptach op in abstract result pass

Update the call conversion pattern to support fir.dispatch
operation as well. The first operand of fir.dispatch op is always the
polymoprhic object. The pass_arg_pos attribute needs to be shifted when
the result is added as argument.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D138799