[X86][AVX512] Use the proper load/store for AVX512 registers.

When loading or storing AVX512 registers we were not using the AVX512
variant of the load and store for VR128 and VR256 like registers.
Thus, we ended up with the wrong encoding and actually were dropping the
high bits of the instruction. The result was that we load or store the
wrong register. The effect is visible only when we emit the object file
directly and disassemble it. Then, the output of the disassembler does
not match the assembly input.

This is related to llvm.org/PR27481.

llvm-svn: 269001