[mlir] Fix a warning

This patch fixes:

  mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp:1304:3: error: default
  label in switch which covers all enumeration values
  [-Werror,-Wcovered-switch-default]
diff --git a/mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp b/mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp
index 4404040..18453aa 100644
--- a/mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp
+++ b/mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp
@@ -1301,8 +1301,6 @@
     return CVT_TO_F6X2_ID_IMPL(e2m3x2, hasRelu);
   case NVVM::CVTFP6Type::E3M2:
     return CVT_TO_F6X2_ID_IMPL(e3m2x2, hasRelu);
-  default:
-    llvm_unreachable("Invalid CVTFP6Type for CvtToF6x2Op");
   }
 }