[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@262244
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