target/ppc: Move vexts[bhw]2[wd] to decodetree

Move the following instructions to decodetree:
vextsb2w: Vector Extend Sign Byte To Word
vextsh2w: Vector Extend Sign Halfword To Word
vextsb2d: Vector Extend Sign Byte To Doubleword
vextsh2d: Vector Extend Sign Halfword To Doubleword
vextsw2d: Vector Extend Sign Word To Doubleword

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Lucas Coutinho <lucas.coutinho@eldorado.org.br>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20220225210936.1749575-8-matheus.ferst@eldorado.org.br>
Signed-off-by: Cédric Le Goater <clg@kaod.org>