arch-power: Refactor arithmetic instructions

arch-power: Refactor arithmetic instructions

This changes the base classes for integer arithmetic
instructions and introduces two new classes that are used
to distinguish between instructions using register and
immediate operands.

Decoding has also been consolidated using formats that can
generate code after determining if an instruction records
carry and overflow and also if it records the nature of the
result, i.e. lesser than, greater than or equal to zero.
However, for multiply and divide instructions, the code to
determine if an overflow has occurred has been moved to the
instruction definition itself. The formats have also been
updated to make use of the new base classes.

Change-Id: I23d70ac4bad4d25d876308db0b3564c092bf574c
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>