[RISCV] Check type is legal before combining mgather to vlse intrinsic

Otherwise we will crash since target intrinsics don't have their types
legalized. Let the mgather get legalized first, then do the combine on the
legal type.
Fixes llvm#81098

Co-authored-by: <[email protected]>

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -15812,7 +15812,7 @@ SDValue RISCVTargetLowering::PerformDAGCombine(SDNode *N,
           MGN->getMemOperand(), IndexType, MGN->getExtensionType());
 
     if (Index.getOpcode() == ISD::BUILD_VECTOR &&
-        MGN->getExtensionType() == ISD::NON_EXTLOAD) {
+        MGN->getExtensionType() == ISD::NON_EXTLOAD && isTypeLegal(VT)) {
       if (std::optional<VIDSequence> SimpleVID = isSimpleVIDSequence(Index);
           SimpleVID && SimpleVID->StepDenominator == 1) {
         const int64_t StepNumerator = SimpleVID->StepNumerator;