[AVX-512] Simplify X86InstrInfo::copyPhysReg for 128/256-bit vectors with...

[AVX-512] Simplify X86InstrInfo::copyPhysReg for 128/256-bit vectors with AVX512, but not VLX. We should use the VEX opcodes and trust the register allocator to not use the extended XMM/YMM register space.

Previously we were extending to copying the whole ZMM register. The register allocator shouldn't use XMM16-31 or YMM16-31 in this configuration as the instructions to spill them aren't available.

llvm-svn: 280648