RFR: Check BS type in immByteMapBase predicate

[Roman Kennke]

In aarch64, we have an instruction in aarch64.ad that blindly casts the 
current BarrierSet to CardTableModRefBS, and uses this in the predicate 
to generate an immediate load if the operand matches the byte_map_base 
of the CTMRBS. However, when used with a GC that doesn't derive its BS 
from the CTMRBS, it reads some random thrash and inserts the special 
instruction sequence (adrp+movk) on immediate loads that happen to match 
whatever is in the imaginary byte_map_base... This eventually leads to 
corrupted heap. The fix is to check the BS type in the predicate too:


http://cr.openjdk.java.net/~rkennke/aarch64-ctmrbs/webrev.00/src/hotspot/cpu/aarch64/aarch64.ad.udiff.html

Test: hotspot_gc_shenandoah on aarch64

I intend to push backports of this to 9 and 8 too. Do I need extra 
reviews for those?

Ok?