arch-power: Refactor branch instructions

arch-power: Refactor branch instructions

This changes the base classes for branch instructions and
switches to two high-level classes for unconditional and
conditional branches. The conditional branches are further
classified based on whether they use an immediate field or
a register for determining the target address.

Decoding has also been consolidated using formats that can
generate code after determining if an instruction branches
to an absolute address or a PC-relative address, or if it
implicitly sets the return address by looking at the AA and
LK bits.

Change-Id: I5fa7db7b6693586b4ea3c71e5cad8a60753de29c
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>