arch-power: Reorder instruction decoding logic

arch-power: Reorder instruction decoding logic

This reorders the decoding logic based on the category of
instructions. The ordering applied here is roughly in line
with the Power ISA manual which is as follows:
  * Branch facility instructions
      * Branch instructions
      * Condition Register instructions
      * System Call instructions
  * Fixed-point facility instructions
      * Load instructions
      * Store instructions
      * Arithmetic instructions
      * Compare instructions
      * Logical instructions
      * Rotate and Shift instructions
      * Move To/From System Register instructions
  * Floating-point facility instructions
      * Load instructions
      * Store instructions
      * Arithmetic instructions
      * Move instructions
      * Rounding and Conversion instructions
      * Compare instructions
      * Status and Control Register instructions

Change-Id: Icfb57c5e442a959e502222222b84289d8e74ecbf
Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>