RFR (XS): 8007708: compiler/6855215 assert(VM_Version::supports_sse4_2())

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/8007708/webrev/

The test runs with -XX:UseSSE=0 flag and as result 
VM_Version::supports_sse4_2() is false.

The code which generates assembler for string_compare intrinsic does not 
check UseSSE42Intrinsics flag when UseAVX > 1 but it uses sse4.2 
instruction (pcmpestri).

Added missing check. I also added missing avx2 assert for vpermq 
instruction.

Ran all compiler regression tests with -Xcomp -XX:UseSSE=0 on avx2 machine.

Thanks,
Vladimir