[ISel] Fix another crash in new FMA DAG combine (#67818)

Following on from D135150, this patch fixes another crash caused by this
DAG combine:

fadd (fma A, B, (fmul C, D)), E --> fma A, B, (fma C, D, E)

The combine calls ReplaceAllUsesOfValueWith to replace (fmul C, D) with
(fma C, D, E). This can cause nodes to get CSEd. In D135150 the problem
was that the (fma C, D, E) node got CSEd away. In this new case, the
problem is that the outer fadd node gets CSEd away. To fix it we have
to return SDValue(N, 0) from the combine and be careful not to add a
deleted node to the worklist.