8321215: Incorrect x86 instruction encoding for VSIB addressing mode

Reviewed-by: shade, thartmann

src/hotspot/cpu/x86/assembler_x86.hpp

@@ -316,7 +316,7 @@ class Address {
   }
 
   bool xmmindex_needs_rex() const {
-    return _xmmindex->is_valid() && _xmmindex->encoding() >= 8;
+    return _xmmindex->is_valid() && ((_xmmindex->encoding() & 8) == 8);
   }
 
   relocInfo::relocType reloc() const { return _rspec.type(); }