target/ppc: Implement Vector Extract Double to VSR using GPR index insns

target/ppc: Implement Vector Extract Double to VSR using GPR index insns

Implement the following PowerISA v3.1 instructions:
vextdubvlx: Vector Extract Double Unsigned Byte to VSR using
            GPR-specified Left-Index
vextduhvlx: Vector Extract Double Unsigned Halfword to VSR using
            GPR-specified Left-Index
vextduwvlx: Vector Extract Double Unsigned Word to VSR using
            GPR-specified Left-Index
vextddvlx: Vector Extract Double Doubleword to VSR using
           GPR-specified Left-Index
vextdubvrx: Vector Extract Double Unsigned Byte to VSR using
            GPR-specified Right-Index
vextduhvrx: Vector Extract Double Unsigned Halfword to VSR using
            GPR-specified Right-Index
vextduwvrx: Vector Extract Double Unsigned Word to VSR using
            GPR-specified Right-Index
vextddvrx: Vector Extract Double Doubleword to VSR using
           GPR-specified Right-Index

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Luis Pires <luis.pires@eldorado.org.br>
Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20211104123719.323713-10-matheus.ferst@eldorado.org.br>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>