Also consider EmitMeshTasksEXT terminator in spirv-opt. (#4930)

Not sure why this is repeated for validation and optimizer, but ...
diff --git a/source/opcode.cpp b/source/opcode.cpp
index e3797b7..88c5c51 100644
--- a/source/opcode.cpp
+++ b/source/opcode.cpp
@@ -453,6 +453,7 @@
     case SpvOpTerminateInvocation:
     case SpvOpTerminateRayKHR:
     case SpvOpIgnoreIntersectionKHR:
+    case SpvOpEmitMeshTasksEXT:
       return true;
     default:
       return false;