Allow use of the arm64 unnamed register form

Allow use of the arm64 unnamed register form

On arm64 all registers have a name that encodes op0, op1, CRn, CRm, and
op2 that are used to encode the register in the instruction. As some
registers we need to access may not be supportedby older compilers, or
are only supported when specific extensions are enabled support this
alternative form.

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