[NVPTX] Add unreachable for TMA Inst Printer

This patch adds the llvm_reachable() for TMA
reduction opcode printer method.

We had this inside the default-case leading to
warning below (and hence was removed):
error: default label in switch which covers all enumeration values
       [-Werror,-Wcovered-switch-default]

Signed-off-by: Durgadoss R <[email protected]>

Author: durga4github

llvm/lib/Target/NVPTX/MCTargetDesc/NVPTXInstPrinter.cpp

@@ -450,4 +450,6 @@ void NVPTXInstPrinter::printTmaReductionMode(const MCInst *MI, int OpNum,
     O << ".xor";
     return;
   }
+  llvm_unreachable(
+      "Invalid Reduction Op in printCpAsyncBulkTensorReductionMode");
 }