transform: fix func lowering assertion failure

The func-lowering pass has started to fail in the dev branch, probably
as a result of replacing the ConstPropagation pass with the IPSCCP pass.
This commit makes the code a bit more robust and should be able to
handle all possible cases (not just ptrtoint).

transform/func-lowering.go

@@ -118,12 +118,18 @@ func LowerFuncValues(mod llvm.Module) {
 						continue
 					}
 					for _, funcValueWithSignatureGlobal := range getUses(funcValueWithSignatureConstant) {
+						id := llvm.ConstInt(uintptrType, uint64(fn.id), false)
 						for _, use := range getUses(funcValueWithSignatureGlobal) {
-							if ptrtoint.IsAConstantExpr().IsNil() || ptrtoint.Opcode() != llvm.PtrToInt {
-								panic("expected const ptrtoint")
+							// Try to replace uses directly: most will be
+							// ptrtoint instructions.
+							if !use.IsAConstantExpr().IsNil() && use.Opcode() == llvm.PtrToInt {
+								use.ReplaceAllUsesWith(id)
 							}
-							use.ReplaceAllUsesWith(llvm.ConstInt(uintptrType, uint64(fn.id), false))
 						}
+						// Remaining uses can be replaced using a ptrtoint.
+						// In my quick testing, this doesn't really happen in
+						// practice.
+						funcValueWithSignatureGlobal.ReplaceAllUsesWith(llvm.ConstIntToPtr(id, funcValueWithSignatureGlobal.Type()))
 					}
 				}
 			}