RFR: Fix broken asserts in ShenandoahSharedEnumFlag

[Roman Kennke]

We assert this in ShenandoahSharedEnumFlag:

assert (v < (1 << sizeof(ShenandoahSharedValue)), "sanity")

However, this only works for values 0 and 1. I propose we change it to
match the same asserts elsewhere in the same file:

assert (v < (sizeof(ShenandoahSharedValue) * CHAR_MAX), "sanity")

http://cr.openjdk.java.net/~rkennke/sharedvalue-fix-assert/webrev.00/

Testing: hotspot_gc_shenandoah ok