arch-power: Fix shift instructions

Now that 64-bit registers are being used, the instructions
must use only the lower word of the operand to be shifted.
This fixes the following instructions.
  * Shift Left Word (slw[.])
  * Shift Right Word (srw[.])
  * Shift Right Algebraic Word (sraw[.])
  * Shift Right Algebraic Word Immediate (srawi[.])

Change-Id: Ibc3124b9e3a8660b0ff9d0178218e34bcc028310
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>