[X86] Make WRPKRU/RDPKRU pass -verify-machineinstrs

[X86] Make WRPKRU/RDPKRU pass -verify-machineinstrs

The original implementation attempted to zero registers using
XOR %foo, %foo.  This is problematic because it constitutes a
read-modify-write of a register which might not be defined.

Instead, use MOV32r0 to avoid these problems; expandPostRAPseudo does
the right thing here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274024 91177308-0d34-0410-b5e6-96231b3b80d8