Revert "[TableGen] Use cast instead of dyn_cast where result isn't checked before being dereferenced."

Turns out I misread the parentheses. Though I'm pretty sure its always a RecordRecTy and non of the callers really seem to expect null. But until I'm completely sure I'm going to revert this.

llvm-svn: 235469
diff --git a/llvm/lib/TableGen/Record.cpp b/llvm/lib/TableGen/Record.cpp
index b81254b..bdafbbc 100644
--- a/llvm/lib/TableGen/Record.cpp
+++ b/llvm/lib/TableGen/Record.cpp
@@ -345,7 +345,7 @@
 }
 
 RecordRecTy *RecordRecTy::get(Record *R) {
-  return cast<RecordRecTy>(R->getDefInit()->getType());
+  return dyn_cast<RecordRecTy>(R->getDefInit()->getType());
 }
 
 std::string RecordRecTy::getAsString() const {