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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178468 91177308-0d34-0410-b5e6-96231b3b80d8