Refactor the Power select evaluators

The Power evaluators for *select opcodes have been significantly
refactored to be much simpler. Additionally a number of new
optimizations have been added to reduce the overhead of these opcodes:

1. Selector nodes which are compares are now evaluated directly to a
   condition register to avoid the overhead of evaluating them and then
   comparing the result against 0.
2. The register for one of the operands is reused if possible to avoid
   an extra mr/fmr instruction.

Signed-off-by: Ben Thomas <ben@benthomas.ca>