arch-power: Fix rotate instructions

arch-power: Fix rotate instructions

Now that 64-bit registers are being used, the rotation
operation changes for words. Instead of just rotating
the lower word of the operand, the lower word is first
duplicated in the upper word and then rotated. This
fixes the following instructions.
  * Rotate Left Word Immediate then And with Mask (rlwinm[.])
  * Rotate Left Word then And with Mask (rlwnm[.])
  * Rotate Left Word Immediate then Mask Insert (rlwimi[.])

Change-Id: Ic743bceb8bafff461276984ecc999dedc1f94e9f
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>