RFR (XS): 8022993: Convert MAX_UNROLL constant to LoopMaxUnroll C2 flag

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/8022993/webrev/

C2 has loop max unroll limit hard coded. It limits size of vectors (byte 
vectors) which are used in AVX instructions.
Replace MAX_UNROLL constant with new C2 LoopMaxUnroll flag.

Thanks,
Vladimir