DebugInfo: Assert dbg.declare/value insts are valid

Remove early returns for when `getVariable()` is null, and just assert
that it never happens.  The Verifier already confirms that there's a
valid variable on these intrinsics, so we should assume the debug info
isn't broken.  I also updated a check for a `!dbg` attachment, which the
Verifier similarly guarantees.

llvm-svn: 235400
diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
index 5ffb826..dafbc63 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -1088,8 +1088,8 @@
   }
   case Intrinsic::dbg_declare: {
     const DbgDeclareInst *DI = cast<DbgDeclareInst>(II);
-    DIVariable DIVar = DI->getVariable();
-    if (!DIVar || !FuncInfo.MF->getMMI().hasDebugInfo()) {
+    assert(DI->getVariable() && "Missing variable");
+    if (!FuncInfo.MF->getMMI().hasDebugInfo()) {
       DEBUG(dbgs() << "Dropping debug info for " << *DI << "\n");
       return true;
     }