[NFC][PowerPC] Style and ordering changes for PPCInstrP10.td

Renamed the two classes 8LS_DForm_R_SI34_RTA5 and 8LS_DForm_R_SI34_XT6_RA5 to
8LS_DForm_R_SI34_RTA5_MEM and 8LS_DForm_R_SI34_XT6_RA5_MEM because the
instructions that use the classes use memory reads/writes.

Moved the instruction defs up closer to the classes.
Removed unnecessary whitespace.