[ISel] Fix crash in new FMA DAG combine

Fix a crash in the FMA combine added by D132837 and amended by D134810.
In cases where the newly created node could be folded, the combiner
would fail this assertion:

llc: DAGCombiner.cpp:268: void (anonymous namespace)::DAGCombiner::AddToWorklist(llvm::SDNode *): Assertion `N->getOpcode() != ISD::DELETED_NODE && "Deleted Node added to Worklist"' failed.

Differential Revision: https://reviews.llvm.org/D135150