[Vector API] Fix an AVX512 crash after JDK-8223347

# Reproduce
Run jdk/incubator/vector/Short64VectorTests.java on AVX512 machines

# Symptom
```
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/jdk/src/hotspot/cpu/x86/assembler_x86.cpp:8714), pid=96989, tid=97022
# assert(((dst_enc < 16 && nds_enc < 16 && src_enc < 16) || (!attributes->is_legacy_mode()))) failed: XMM register should be 0-15
#
# JRE version: OpenJDK Runtime Environment (16.0+4) (fastdebug build 16-internal+4-adhoc..jdk)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 16-internal+4-adhoc..jdk, mixed mode, sharing, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x63259f] Assembler::vex_prefix_and_encode(int, int, int, Assembler::VexSimdPrefix, Assembler::VexOpcode, InstructionAttr*) [clone .constprop.1]+0x21f
#
```