This change fixes a few bugs in the ppc64 port.
* Expand of max/min nodes in .ad file relied on 64-bit sign extend
but placed the values in 32-bit registers. After SpillCopies
the upper bits were lost.
* Expand of decodeNKlass used wrong node.
* Implement missing support for method handles in generate_throw_exception.
* Some small optimizations and cleanups.