[X86] Use VR512_0_15RegClass intead of VR512RegClass in X86VZeroUpper.

This pass is only concerned with ZMM0-15 and YMM0-15. For YMM
we use VR256 which only contains YMM0-15, but for ZMM we were
using VR512 which contains ZMM0-31. Using VR512_0_15 is more
correct.

Given that the ABI and register allocator will use registers in
order, its unlikely that register from 16-31 would be used
without also using 0-15. So this probably doesn't functionally
matter.

llvm-svn: 372933