[RISCV] Check type is legal before combining mgather to vlse intrinsic (llvm#81107)

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#81088

Co-authored-by: Craig Topper <craig.topper@sifive.com>

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -15833,7 +15833,7 @@ 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;