arch-power: Fix fixed-point word shift instructions

arch-power: Fix fixed-point word shift instructions

This fixes the following shift instructions:
  * Shift Left Word (slw[.])
  * Shift Right Word (srw[.])
  * Shift Right Algebraic Word (sraw[.])
  * Shift Right Algebraic Word Immediate (srawi[.])

For 64-bit execution, these instructions should perform
shift operations on only the lower order 32 bits of the
source register instead of all 64 bits.

This also fixes disassembly generation for all of the above.

Change-Id: I18871486d74969244d474eaf0f9d810f06faf50a
Signed-off-by: Sandipan Das <sandipan@linux.vnet.ibm.com>