In review of bbde6b, I had originally proposed that we support the
legacy text format. As review evolved, it bacame clear this had been a
bad idea (too much complexity), but in order to let that patch finally
move forward, I approved the change with the variant. This change undoes
the variant, and updates all the tests to just use the array form.
As reported from a failing firefox build, we can sometimes get frame
indices with negative offsets from a t2LDRi8. This adds support for
them, to prevent the crash.