[WASM] Prevent casting `undef` to `CosntantSDNode`

[Peter]

Summary

WebAssembly tries to cast an undef to CosntantSDNode during LowerAccessVectorElement.
These operations will trigger an assertion error in cast.
To avoid this issue, we prevent casting, and abort the lowering operation.
A unit test is also included.

This patch fixes pr61828

[Peter]

precommit format changes

[tlively]

Thanks!

llvm/lib/Target/WebAssembly/WebAssemblyISelLowering.cpp

return DAG.getBuildVector(Op->getValueType(0), DL,
                             {MakeLane(0), MakeLane(1)});
 }
 
 SDValue
 WebAssemblyTargetLowering::LowerAccessVectorElement(SDValue Op,
                                                     SelectionDAG &DAG) const {
   // Allow constant lane indices, expand variable lane indices
   SDNode *IdxNode = Op.getOperand(Op.getNumOperands() - 1).getNode();
-  if (isa<ConstantSDNode>(IdxNode) || IdxNode->isUndef()) {
+  if (isa<ConstantSDNode>(IdxNode)) {
     // Ensure the index type is i32 to match the tablegen patterns
     uint64_t Idx = cast<ConstantSDNode>(IdxNode)->getZExtValue();
     SmallVector<SDValue, 3> Ops(Op.getNode()->ops());
     Ops[Op.getNumOperands() - 1] =
         DAG.getConstant(Idx, SDLoc(IdxNode), MVT::i32);
     return DAG.getNode(Op.getOpcode(), SDLoc(Op), Op.getValueType(), Ops);
   }
   // Perform default expansion

llvm/test/CodeGen/WebAssembly/pr61828.ll

+; RUN: llc < %s -mattr=+simd128 -mtriple=wasm64
+
+define void @foo(i64 %i0, i64 %i1, ptr %p) {
+  %B4 = urem i64 %i0, %i0
+  %B5 = udiv i64 %i1, %B4
+  %I = insertelement <4 x float> <float 0.25, float 0.25, float 0.25, float 0.25>, float 0.5, i64 %B5
+  store <4 x float> %I, ptr %p
+  ret void
+}