[SelectionDAG] Use LAST_INTEGER_VALUETYPE instead of i64 (llvm#98299)

When looking for a largest legal integer type for a target
`TargetLowering::findOptimalMemOpLowering` assumes that `MVT::i64` is
the largets possible integer type. The patch removes this assumption and
uses `MVT::LAST_INTEGER_VALUETYPE` instead.

Author: akiramenai

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -209,15 +209,15 @@ bool TargetLowering::findOptimalMemOpLowering(
     // Use the largest integer type whose alignment constraints are satisfied.
     // We only need to check DstAlign here as SrcAlign is always greater or
     // equal to DstAlign (or zero).
-    VT = MVT::i64;
+    VT = MVT::LAST_INTEGER_VALUETYPE;
     if (Op.isFixedDstAlign())
       while (Op.getDstAlign() < (VT.getSizeInBits() / 8) &&
              !allowsMisalignedMemoryAccesses(VT, DstAS, Op.getDstAlign()))
         VT = (MVT::SimpleValueType)(VT.getSimpleVT().SimpleTy - 1);
     assert(VT.isInteger());
 
     // Find the largest legal integer type.
-    MVT LVT = MVT::i64;
+    MVT LVT = MVT::LAST_INTEGER_VALUETYPE;
     while (!isTypeLegal(LVT))
       LVT = (MVT::SimpleValueType)(LVT.SimpleTy - 1);
     assert(LVT.isInteger());