Treat PPCISD::STFIWX like the memory opcode that it is

PPCISD::STFIWX is really a memory opcode, and so it should come after
FIRST_TARGET_MEMORY_OPCODE, and we should use DAG.getMemIntrinsicNode to create
nodes using it.

No functionality change intended (although there could be optimization benefits
from preserving the MMO information).