[mlir] Correctly indent block terminators

These were non-indented, which I thought was deliberate until Chris corrected me in cl/207115253 :)

PiperOrigin-RevId: 207246887
diff --git a/lib/IR/AsmPrinter.cpp b/lib/IR/AsmPrinter.cpp
index b2ebaa6..c53c333 100644
--- a/lib/IR/AsmPrinter.cpp
+++ b/lib/IR/AsmPrinter.cpp
@@ -917,6 +917,7 @@
     os << '\n';
   }
 
+  os << "  ";
   print(block->getTerminator());
   os << '\n';
 }