The source tree currently contains an implementation of instruction scheduling in the org.jikesrvm.compilers.opt.instrsched package. The implementation is PPC-specific and deactivated by default. Enabling the phase shows it to be broken: it fails with an NPE in OperatorClass.
Therefore, the best option is to remove the instruction scheduler. This will also help to remove scratch fields and make some tiny progress on the removal of ArchitectureSpecific as the instruction scheduler relies on code generated during the build process.