RFR(S): x86: 8190494: fix for different results with UseAVX=3 when calling AVX-512 native function via JNI.

[Vladimir Ivanov]

> http://cr.openjdk.java.net/~vdeshpande/8190494/webrev.00/

Thanks, Vivek. I verified that the reported problem goes away with the fix.

src/hotspot/cpu/x86/macroAssembler_x86.cpp

+  // Reset k1 to 0xffff.
+  if (VM_Version::supports_evex()) {
+    push(rcx);
+    movl(rcx, 0xffff);
+    kmovwl(k1, rcx);
+    pop(rcx);
+  }

Why do you preserve rcx which is already caller-saved and shouldn't 
contain anything valuable after a JNI call?

Best regards,
Vladimir Ivanov