[14] RFR (S): 8231430: C2: Memory stomp in max_array_length() for T_ILLEGAL type

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8231430/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8231430

There's a memory stomp happening in max_array_length() for T_ILLEGAL 
type. T_ILLEGAL type arises as an element basic type for a merge of 2 
primitive arrays (bottom[]). max_array_length() does some input 
normalization (T_ILLEGAL => T_BYTE), but first it acquires a reference 
to the a cache slot which is out-of-bounds (T_ILLEGAL = 99 vs T_CONFLICT 
= 19).

I was able to reproduce the problem as a corruption of one of the OOPs 
in Universe::_mirrors array which happened to be put close enough to 
max_array_length_cache in memory.

I propose to completely remove the cache. 
arrayOopDesc::max_array_length() doesn't look too expensive and the 
method is not used on a hot path anywhere.

Also, I put an assert for T_VOID, T_CONFLICT, T_NARROWKLASS cases, but 
left the logic there (=> T_BYTE) to get more testing before removing them.

Testing: hs-precheckin-comp, tier1-5.

Best regards,
Vladimir Ivanov