Allow one layer of unbalanced if to be inlined

[limepoutine]

This now gives small performance benefits with std.random unittests (dmd -inline -main -O -unittest -version=StdUnittest random.d).

Benchmark 1: ./random_master
  Time (mean ± σ):     451.5 ms ±   5.1 ms    [User: 450.3 ms, System: 3.1 ms]
  Range (min … max):   443.7 ms … 465.3 ms    20 runs

Benchmark 2: ./random_pr
  Time (mean ± σ):     444.1 ms ±   4.1 ms    [User: 442.9 ms, System: 3.1 ms]
  Range (min … max):   438.6 ms … 455.5 ms    20 runs
 
Summary
  ./random_pr ran
    1.02 ± 0.01 times faster than ./random_master