[MIR] Speedup parsing (up to 1000x+)

[evgeny777]

Summary

Patch eliminates string length calculation when lexing a token. It lowers parsing time from 15 minutes to few seconds for 4MB MIR file on Ryzen7 4800H

[dblaikie]

llvm/lib/CodeGen/MIRParser/MIParser.cpp

566

drop_front(SkipChar) might be simpler/clearer?

llvm/lib/CodeGen/MIRParser/MIParser.cpp

 
 MIParser::MIParser(PerFunctionMIParsingState &PFS, SMDiagnostic &Error,
                    StringRef Source)
     : MF(PFS.MF), Error(Error), Source(Source), CurrentSource(Source), PFS(PFS)
 {}
 
 void MIParser::lex(unsigned SkipChar) {
   CurrentSource = lexMIToken(
-      CurrentSource.data() + SkipChar, Token,
+      CurrentSource.slice(SkipChar, StringRef::npos), Token,
       [this](StringRef::iterator Loc, const Twine &Msg) { error(Loc, Msg); });
 }
 
 bool MIParser::error(const Twine &Msg) { return error(Token.location(), Msg); }
 
 bool MIParser::error(StringRef::iterator Loc, const Twine &Msg) {
   const SourceMgr &SM = *PFS.SM;
   assert(Loc >= Source.data() && Loc <= (Source.data() + Source.size()));