[SelectionDAG] Kill last uses of getAtomic w/o a MMO operand [NFC]

The helper function was used by only two callers, and largely ended up providing distinct functionality based on optional arguments and opcode.  Inline and simply to make the functionality much more clear.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353977 91177308-0d34-0410-b5e6-96231b3b80d8