This patch adds codegen for all Arm9.6-a compare-and-branch
instructions, that operate on full w or x registers. The instruction
variants operating on half-words (cbh) and bytes (cbb) are added in a
subsequent patch.
Since CB doesn't use standard 4-bit Arm condition codes but a reduced
set of conditions, encoded in 3 bits, some conditions are expressed by
modifying operands, namely incrementing or decrementing immediate
operands and swapping register operands. To invert a CB instruction it's
therefore not enough to just modify the condition code which doesn't
play particularly well with how the backend is currently organized. We
therefore introduce a number of pseudos which operate on the standard
4-bit condition codes and lower them late during codegen.
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