[RISCV] Limit VLEN in getOptimalMemOpType to prevent creating invalid MVTs.

We only guarantee that types that are 1024 bytes or smaller exist
in the MVT enum.

Fixes #139075.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -23487,7 +23487,9 @@ EVT RISCVTargetLowering::getOptimalMemOpType(const MemOp &Op,
   // combining will typically form larger LMUL operations from the LMUL1
   // operations emitted here, and that's okay because combining isn't
   // introducing new memory operations; it's just merging existing ones.
-  const unsigned MinVLenInBytes = Subtarget.getRealMinVLen()/8;
+  // NOTE: We limit to 1024 bytes to avoid creating an invalid MVT.
+  const unsigned MinVLenInBytes = std::min(Subtarget.getRealMinVLen()/8, 1024U);
+
   if (Op.size() < MinVLenInBytes)
     // TODO: Figure out short memops.  For the moment, do the default thing
     // which ends up using scalar sequences.