arch-power: Fix extended opcode based decoding
arch-power: Fix extended opcode based decoding

When multiple instructions share the same primary opcode,
the decoder can distinguish between them by looking at the
extended opcode field. However, the length and position of
the extended opcode field can slightly vary depending on
the instruction form.

This ensures that the correct extended opcode fields are
used for decoding such instructions.

Change-Id: I8207568ac975587377abba8a9b221ca3097b8488
Signed-off-by: Sandipan Das <ae8113f64d9c72812e097938d25a8975da69c074@linux.ibm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40885
Reviewed-by: Boris Shingarov <9e9a86074d235a5ecff50c3f2485c8b344969b31@gmail.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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