JIT: flush a register if it won't be used for the rest of the block

This should dramatically reduce code size in the case of blocks with
lots of branches, and certainly doesn't hurt elsewhere either.

This can probably be improved a good bit through smarter tracking of register
usage, e.g. discarding registers that are going to be overwritten, but this
is a good start and should help reduce code size and register pressure.

As part of this, refactor PPCAnalyst to support distinguishing between
float and integer registers (to properly handle instructions that access
both, like floating-point loads and stores).