With the addition of 128-bit system registers to the Arm architecture
starting with Armv9.4-a, a mechanism for manipulating their contents
is introduced with the `msrr' and `mrrs' instruction pair.
These move values from one such 128-bit system register into a pair of
contiguous general-purpose registers and vice-versa, as for example:
msrr ttlb0_el1, x0, x1
mrrs x0, x1, ttlb0_el1
This patch adds the necessary support for these instructions, adding
checks for system-register width by defining a new operand type in the
form of `AARCH64_OPND_SYSREG128' and the `aarch64_sys_reg_128bit_p'
predicate, responsible for checking whether the requested system
register table entry is marked as implemented in the 128-bit mode via
the F_REG_128 flag.
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