Aarch ncg: Optimize immediate use for address calculations

When the offset doesn't fit into the immediate we now just reuse the
general getRegister' code path which is well optimized to compute the
offset into a register instead of a special case for CmmRegOff.

This means we generate a lot less code under certain conditions which is
why performance metrics for these improve.

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Metric Decrease:
    T4801
    T5321FD
    T5321Fun
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