target/arm: Make FEAT_MOPS SET* insns handle Xs == XZR correctly

target/arm: Make FEAT_MOPS SET* insns handle Xs == XZR correctly

Most of the registers used by the FEAT_MOPS instructions cannot use
31 as a register field value; this is CONSTRAINED UNPREDICTABLE to
NOP or UNDEF (we UNDEF).  However, it is permitted for the "source
value" register for the memset insns SET* to be 31, which (as usual
for most data-processing insns) means it should be the zero register
XZR. We forgot to handle this case, with the effect that trying to
set memory to zero with a "SET* Xd, Xn, XZR" sets the memory to
the value that happens to be in the low byte of SP.

Handle XZR when getting the SET* data value from the register file.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20231030174000.3792225-4-peter.maydell@linaro.org