Handle local struct pointer arithmetic

The assert introduced in dotnet#23570 was overly aggressive, and didn't account for the fact that pointer arithmetic can exist in the IL, not just introduced by morph.

Fix #23693

src/jit/importer.cpp

@@ -10752,19 +10752,11 @@ void Compiler::impImportBlockCode(BasicBlock* block)
  }
  else if (lhs->OperIsBlk())
  {
- // Note that, prior to morph, we will only see ADDR(LCL_VAR) for any assignment to
- // a local struct. We should never see LCL_VAR_ADDR or ADD(ADDR(LCL_VAR) + CNS).
- // Other local struct references (e.g. FIELD or more complex pointer arithmetic)
- // will cause the stack to be spilled.
- GenTree* addr = lhs->AsBlk()->Addr();
- if (addr->OperIs(GT_ADDR) && addr->gtGetOp1()->OperIs(GT_LCL_VAR))
- {
- lclVar = addr->gtGetOp1()->AsLclVarCommon();
- }
- else
- {
- assert(addr->IsLocalAddrExpr() == nullptr);
- }
+ // Check for ADDR(LCL_VAR), or ADD(ADDR(LCL_VAR),CNS_INT))
+ // (the latter may appear explicitly in the IL).
+ // Local field stores will cause the stack to be spilled when
+ // they are encountered.
+ lclVar = lhs->AsBlk()->Addr()->IsLocalAddrExpr();
  }
  if (lclVar != nullptr)
  {