[MC] Remove SetUsed on isUndefined and getFragment

Due to the known limitations of .set reassignment (see
https://sourceware.org/PR288), we use diagnostics to reject patterns
that could lead to errors (ae7ac01 2009-06)).

This code gets refined multiple times, see:

* 9b4a824 (2010-05) `IsUsedInExpr`
* 46c79ef (2010-11) renamed `IsUsedInExpr` to `IsUsed`

The related `SetUsed` bit seems unnecessary nowadays.

Author: MaskRay

llvm/include/llvm/MC/MCSymbol.h

@@ -257,9 +257,7 @@ class MCSymbol {
   }
 
   /// isUndefined - Check if this symbol undefined (i.e., implicitly defined).
-  bool isUndefined(bool SetUsed = true) const {
-    return getFragment(SetUsed) == nullptr;
-  }
+  bool isUndefined() const { return getFragment() == nullptr; }
 
   /// isAbsolute - Check if this is an absolute symbol.
   bool isAbsolute() const {
@@ -395,7 +393,7 @@ class MCSymbol {
     return SymbolContents == SymContentsTargetCommon;
   }
 
-  MCFragment *getFragment(bool SetUsed = true) const {
+  MCFragment *getFragment(bool SetUsed = false) const {
     if (Fragment || !isVariable() || isWeakExternal())
       return Fragment;
     // If the symbol is a non-weak alias, get information about

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -5284,9 +5284,9 @@ bool AsmParser::parseDirectiveIfdef(SMLoc DirectiveLoc, bool expect_defined) {
     MCSymbol *Sym = getContext().lookupSymbol(Name);
 
     if (expect_defined)
-      TheCondState.CondMet = (Sym && !Sym->isUndefined(false));
+      TheCondState.CondMet = (Sym && !Sym->isUndefined());
     else
-      TheCondState.CondMet = (!Sym || Sym->isUndefined(false));
+      TheCondState.CondMet = (!Sym || Sym->isUndefined());
     TheCondState.Ignore = !TheCondState.CondMet;
   }
 
@@ -6362,8 +6362,7 @@ bool parseAssignmentExpression(StringRef Name, bool allow_redef,
     // FIXME: Diagnose assignment to protected identifier (e.g., register name).
     if (Value->isSymbolUsedInExpression(Sym))
       return Parser.Error(EqualLoc, "Recursive use of '" + Name + "'");
-    else if (Sym->isUndefined(/*SetUsed*/ false) && !Sym->isUsed() &&
-             !Sym->isVariable())
+    else if (Sym->isUndefined() && !Sym->isUsed() && !Sym->isVariable())
       ; // Allow redefinitions of undefined symbols only used in directives.
     else if (Sym->isVariable() && !Sym->isUsed() && allow_redef)
       ; // Allow redefinitions of variables that haven't yet been used.

llvm/lib/MC/MCParser/MasmParser.cpp

@@ -4761,7 +4761,7 @@ bool MasmParser::parseDirectiveIfdef(SMLoc DirectiveLoc, bool expect_defined) {
         is_defined = true;
       } else {
         MCSymbol *Sym = getContext().lookupSymbol(Name.lower());
-        is_defined = (Sym && !Sym->isUndefined(false));
+        is_defined = (Sym && !Sym->isUndefined());
       }
     }
 
@@ -4882,7 +4882,7 @@ bool MasmParser::parseDirectiveElseIfdef(SMLoc DirectiveLoc,
         is_defined = true;
       } else {
         MCSymbol *Sym = getContext().lookupSymbol(Name);
-        is_defined = (Sym && !Sym->isUndefined(false));
+        is_defined = (Sym && !Sym->isUndefined());
       }
     }
 
@@ -5052,7 +5052,7 @@ bool MasmParser::parseDirectiveErrorIfdef(SMLoc DirectiveLoc,
       IsDefined = true;
     } else {
       MCSymbol *Sym = getContext().lookupSymbol(Name);
-      IsDefined = (Sym && !Sym->isUndefined(false));
+      IsDefined = (Sym && !Sym->isUndefined());
     }
   }