AArch64: Extend support for different loop variants

[hanno-becker]

The AArch64 model currently only supports a very limited set of loop forms. To expand the range of programs that SLOTHY can be applied to without major modification, we should expand this.

For concrete examples, see the loop forms used in the assembly routines here: https://github.com/intel/isa-l/tree/master/erasure_code/aarch64

For example:

add     cnt, cnt, #128        # Increment position counter
...
cmp     cnt, threshold                  # Compare position vs length
ble     LABEL                       # Branch if less or equal (signed)

or

ldr/str   ..., [cnt], #128        # Increment position counter
...
cmp     cnt, threshold              # Compare position vs length
ble     LABEL                # Branch if less or equal (signed)

where threshold is fixed in either case.

@dop-amin How would you approach this?

[dop-amin]

@hanno-becker I've addressed this in #333. Please take a look if that suits your needs.