[X86] Use ISD::MERGE_VALUES to return from lowerAtomicArith instead of calling...

[X86] Use ISD::MERGE_VALUES to return from lowerAtomicArith instead of calling ReplaceAllUsesOfValueWith and returning SDValue().

Returning SDValue() makes the caller think that nothing happened and it will
end up executing the Expand path. This generates extra nodes that will need to
be pruned as dead code.

Returning an ISD::MERGE_VALUES will tell the caller that we'd like to make a
change and it will take care of replacing uses. This will prevent falling into
the Expand path.

llvm-svn: 360627