[ARM] Fix MVEFloatOps check on creating VCVTN (#79291)

In the past PerformSplittingToNarrowingStores handled both int and float
ops, but since the introduction of MVETRUNC now only operates on float
operations, creating VCVTN nodes. It should be guarded by hasMVEFloatOps
to prevent a failure to select.

Author: davemgreen

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -16803,9 +16803,11 @@ static SDValue PerformSTORECombine(SDNode *N,
     if (SDValue Store = PerformTruncatingStoreCombine(St, DCI.DAG))
       return Store;
 
-  if (Subtarget->hasMVEIntegerOps()) {
+  if (Subtarget->hasMVEFloatOps())
     if (SDValue NewToken = PerformSplittingToNarrowingStores(St, DCI.DAG))
       return NewToken;
+
+  if (Subtarget->hasMVEIntegerOps()) {
     if (SDValue NewChain = PerformExtractFpToIntStores(St, DCI.DAG))
       return NewChain;
     if (SDValue NewToken =