[MergeFunctions] Don't blindly RAUW a GlobalValue with a ConstantExpr.

MergeFunctions uses (through FunctionComparator) a map of GlobalValues
to identifiers because it needs to compare functions and globals
do not have an inherent total order. Thus, FunctionComparator
(through GlobalNumberState) has a ValueMap<GlobalValue *>.

r315852 added a RAUW on globals that may have been previously
encountered by the FunctionComparator, which would replace
a GlobalValue * key with a ConstantExpr *, which is illegal.

This commit adjusts that code path to remove the function being
replaced from the ValueMap as well.

llvm-svn: 316145

llvm/include/llvm/Transforms/Utils/FunctionComparator.h

@@ -78,6 +78,10 @@ public:
     return MapIter->second;
   }
 
+  void erase(GlobalValue *Global) {
+    GlobalNumbers.erase(Global);
+  }
+
   void clear() {
     GlobalNumbers.clear();
   }

llvm/lib/Transforms/IPO/MergeFunctions.cpp

@@ -629,6 +629,9 @@ void MergeFunctions::filterInstsUnrelatedToPDI(
 void MergeFunctions::writeThunk(Function *F, Function *G) {
   if (!G->isInterposable() && !MergeFunctionsPDI) {
     if (G->hasGlobalUnnamedAddr()) {
+      // G might have been a key in our GlobalNumberState, and it's illegal
+      // to replace a key in ValueMap<GlobalValue *> with a non-global.
+      GlobalNumbers.erase(G);
       // If G's address is not significant, replace it entirely.
       Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
       G->replaceAllUsesWith(BitcastF);