[DebugInfo] MIRParser support to handle DW_OP_LLVM_convert ops

[markus]

Summary

https://reviews.llvm.org/D56587 introduced DW_OP_LLVM_convert but forgot to update the MIR parser (came apparent when trying to add a .mir test).

This is a trivial update to the parser.

lib/CodeGen/MIRParser/MIParser.cpp

bool MIParser::parseDIExpression(MDNode *&Expr) {
   if (Token.isNot(MIToken::rparen)) {
     do {
       if (Token.is(MIToken::Identifier)) {
         if (unsigned Op = dwarf::getOperationEncoding(Token.stringValue())) {
           lex();
           Elements.push_back(Op);
           continue;
         }
+        if (unsigned Enc = dwarf::getAttributeEncoding(Token.stringValue())) {
+          lex();
+          Elements.push_back(Enc);
+          continue;
+        }
         return error(Twine("invalid DWARF op '") + Token.stringValue() + "'");
       }
 
       if (Token.isNot(MIToken::IntegerLiteral) ||
           Token.integerValue().isSigned())
         return error("expected unsigned integer");
 
       auto &U = Token.integerValue();