[SelectionDAG] Bail out of mergeTruncStores when not optimizing

With unoptimized code, we may see lots of stores and spend too much time in mergeTruncStores.

Fixes PR51827.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D111596

Author: aeubanks

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

@@ -7296,7 +7296,7 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
   // TODO: If there is evidence that running this later would help, this
   //       limitation could be removed. Legality checks may need to be added
   //       for the created store and optional bswap/rotate.
-  if (LegalOperations)
+  if (LegalOperations || OptLevel == CodeGenOpt::None)
     return SDValue();
 
   // We only handle merging simple stores of 1-4 bytes.