Represent labels as integers for easier offset calculation.

This commit changes the representation of labels from strings to
integers to simplify pass/drop offset calculations. Labels less than -1
represent internal program labels used for offset calculations, while
labels greater than or equal to 0 represent pass/drop labels with an
offset calculated by adding the label value to the program size.

Test: TH
Change-Id: I5e23b0c3d73044fdaf515f673d3766826b4db419
6 files changed