[FPEnv] Fix chain handling regression after 04a86966

Code in getRoot made the assumption that every node in PendingLoads
must always itself have a dependency on the current DAG root node.

After the changes in 04a86966, it turns out that this assumption no
longer holds true, causing wrong codegen in some cases (e.g. stores
after constrained FP intrinsics might get deleted).

To fix this, we now need to make sure that the TokenFactor created
by getRoot always includes the previous root, if there is no implicit
dependency already present.

The original getControlRoot code already has exactly this check,
so this patch simply reuses that code now for getRoot as well.
This fixes the regression.

NFC if no constrained FP intrinsic is present.