Remove implicit fallthrough (#3298)

Fixes #3296

* Make OpReturn its own case fully

source/val/validate_cfg.cpp

diff --git a/source/val/validate_cfg.cpp b/source/val/validate_cfg.cpp
index 1c279f6..1e33e51 100644
--- a/source/val/validate_cfg.cpp
+++ b/source/val/validate_cfg.cpp
@@ -1090,8 +1090,9 @@
         return _.diag(SPV_ERROR_INVALID_CFG, inst)
                << "OpReturn can only be called from a function with void "
                << "return type.";
+      _.current_function().RegisterBlockEnd(std::vector<uint32_t>(), opcode);
+      break;
     }
-    // Fallthrough.
     case SpvOpKill:
     case SpvOpReturnValue:
     case SpvOpUnreachable: