[X86] Move the ATOMIC_LOAD_OP ISel from DAGToDAG to ISelLowering. NFCI.

This is long-standing dirtiness, as acknowledged by r77582:

    The current trick is to select it into a merge_values with
    the first definition being an implicit_def. The proper solution is
    to add new ISD opcodes for the no-output variant.

Doing this before selection will let us combine away some constructs.

Differential Revision: http://reviews.llvm.org/D17659

llvm-svn: 262244