arch-power: Refactor instruction decoding

arch-power: Refactor instruction decoding

This reorders the decoding logic based on the category of
instructions. The ordering scheme used here is roughly in
line with the Power ISA manual as shown below.
  * 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: Ia2d457967bfebb7b20163b56db1cbbe03ac17ceb
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>