[RISCV] atomic_store_nn have a different layout to regular store

We cannot directy reuse the patterns of StPat because for some reason the store
DAG node and the atomic_store_nn DAG nodes put the ptr and the value in
different positions. Currently we attempt to store the address to an address
formed by the value.

Differential Revision: https://reviews.llvm.org/D51217

llvm-svn: 340722