[FPEnv][SelectionDAG] Refactor strict FP node construction

Small refactoring in visitConstrainedFPIntrinsic that should make
it easier to create DAG nodes requiring extra arguments.  That is
the case currently only for STRICT_FP_ROUND, but may be the case
for additional nodes (in particular compares) in the future.

Extracted from the patch for D69281.

NFC.