#500 mcs51/gen.c: extract function emit_add_dptr,emit inline sequence if beneficial

[Oleg Endo]

In mcs51/gen.c there are currently 3 copy-pasta places where it does a dptr increment/decrement operation.

The attached patch extracts it into a function and also emits an inline sequence to add/subtract larger values to/from dptr instead of emitting multiple inc dptr or lcall __decdptr