[TableGen] Replace some dyn_casts followed by an assert with just a regular cast which asserts internally. NFC

llvm-svn: 236087
diff --git a/llvm/lib/TableGen/TGParser.cpp b/llvm/lib/TableGen/TGParser.cpp
index 3730172..9bd6cc6 100644
--- a/llvm/lib/TableGen/TGParser.cpp
+++ b/llvm/lib/TableGen/TGParser.cpp
@@ -112,8 +112,7 @@
     }
 
     // We should have a BitsInit type now.
-    BitsInit *BInit = dyn_cast<BitsInit>(BI);
-    assert(BInit != nullptr);
+    BitsInit *BInit = cast<BitsInit>(BI);
 
     SmallVector<Init *, 16> NewBits(CurVal->getNumBits());