RFR:8144771: AVX3 patch for MacroAssembler::string_compare

[Civlin, Jan]

We would like to contribute AVX3 patch for MacroAssembler::string_compare.

This utilizes 512 bits registers on AVX3 architecture and delivers performance gain (speed-up) on long strings at about x 1.33 and on random string about x 1.22. This was measured vs AVX2 (256 bits registers).


Contributors:
MacroAssembler::string_compare - Jan Civlin.
Rest of code, including all x86 AVX3 extensions  - Michael Berg


Bug-id: https://bugs.openjdk.java.net/browse/JDK-8144771
Webrev:  http://cr.openjdk.java.net/~kvn/8144771/webrev/