target/arm: Implement MVE interleaving loads/stores

target/arm: Implement MVE interleaving loads/stores

Implement the MVE interleaving load/store functions VLD2, VLD4, VST2
and VST4.  VLD2 loads 16 bytes of data from memory and writes to 2
consecutive Qregs; VLD4 loads 16 bytes of data from memory and writes
to 4 consecutive Qregs.  The 'pattern' field in the encoding
determines the offset into memory which is accessed and also which
elements in the Qregs are written to.  (The intention is that a
sequence of four consecutive VLD4 with different pattern values
performs a complete de-interleaving load of 64 bytes into all
elements of the 4 Qregs.) VST2 and VST4 do the same, but for stores.

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