RFR (M): 8005419: Improve intrinsics code performance on x86 by using AVX2

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/8005419/webrev/

Use 256bit (AVX2) vpxor,vptest instructions in String.compareTo() and 
equals() intrinsics.

Added jtreg test.

Ran compiler regression tests and ctw.

Thanks,
Vladimir