[SelectionDAG] Fix NaN regression in fma dag-combine. (#146592)
After 901e1390c9778a191256335d37802bc631c2d183 (#127770), the DAG
combine would transform `fma(x, 0.0, 1.0)` into `1.0` if
`-fp-contract=fast` was enabled, in addition to when 'x' is marked
nnan/ninf.
It's only valid in the latter case, not the former, so delete the extra
condition.