[AVX512] Support mask register in MRMDestReg format

This is necessary for masking vextract*x4.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219359 91177308-0d34-0410-b5e6-96231b3b80d8

utils/TableGen/X86RecognizableInstr.cpp

@@ -592,6 +592,8 @@ void RecognizableInstr::emitInstructionSpecifier() {
            "Unexpected number of operands for MRMDestRegFrm");
 
     HANDLE_OPERAND(rmRegister)
+    if (HasEVEX_K)
+      HANDLE_OPERAND(writemaskRegister)
 
     if (HasVEX_4V)
       // FIXME: In AVX, the register below becomes the one encoded