[codeview] Fix assertion failure introduced in r295354 refactoring

CodeViewDebug sets Asm to nullptr to disable debug info generation.  You
can get a .ll file like no-cus.ll from 'clang -gcodeview -g0', which
happens in the ubsan test suite.

llvm-svn: 302923
diff --git a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
index 1d63e33..826162a 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp
@@ -129,10 +129,9 @@
 }
 
 void DebugHandlerBase::beginFunction(const MachineFunction *MF) {
-  assert(Asm);
   PrevInstBB = nullptr;
 
-  if (!hasDebugInfo(MMI, MF)) {
+  if (!Asm || !hasDebugInfo(MMI, MF)) {
     skippedNonDebugFunction();
     return;
   }