[PPC] cleanup of mayLoad/mayStore flags and memory operands.

1) Explicitly sets mayLoad/mayStore property in the tablegen files on load/store
   instructions.
2) Updated the flags on a number of intrinsics indicating that they write
    memory.
3) Added SDNPMemOperand flags for some target dependent SDNodes so that they
   propagate their memory operand

Review: https://reviews.llvm.org/D28818
llvm-svn: 293200