target/arm: Make VMOV scalar <-> gpreg beatwise for MVE

target/arm: Make VMOV scalar <-> gpreg beatwise for MVE

In a CPU with MVE, the VMOV (vector lane to general-purpose register)
and VMOV (general-purpose register to vector lane) insns are not
predicated, but they are subject to beatwise execution if they
are not in an IT block.

Since our implementation always executes all 4 beats in one tick,
this means only that we need to handle PSR.ECI:
 * we must do the usual check for bad ECI state
 * we must advance ECI state if the insn succeeds
 * if ECI says we should not be executing the beat corresponding
   to the lane of the vector register being accessed then we
   should skip performing the move

Note that if PSR.ECI is non-zero then we cannot be in an IT block.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210617121628.20116-45-peter.maydell@linaro.org