RFR (S): Specialize update-refs for disabled matrix case

[Aleksey Shipilev]

Hi,

This improves performance when partial/matrix is not enabled, for obvious reasons.

Patch:
  http://cr.openjdk.java.net/~shade/shenandoah/update-refs-no-matrix/webrev.01/

Before:
  Concurrent Update Refs      =     4.04 s (a =   269496 us) (n =    15)
     (lvls, us =   169922,   271484,   277344,   279297,   300987)

After:
  Concurrent Update Refs      =     3.56 s (a =   237003 us) (n =    15)
     (lvls, us =   162109,   234375,   238281,   240234,   295872)

Testing: hotspot_gc_shenandoah, benchmarks

Thanks,
-Aleksey