The Armv9.6-A compare-and-branch instructions use a short range 9-bit
immediate value. They do not have a corresponding relocation type in the
ABI. For now we only support them in compact code model, with
diagnostics added in the LongJmp pass to ensure this condition. Some
interesting edge cases we cover:
- function splitting works when target is within or beyond the 1KB range
of those instructions,
- but doesn't work beyond the 128MB limit of the compact code model
- branch inversion works with block reordering so long as the immediate
value adjustments remain in bounds
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