[llvm] Allocate the buffers of the real function in the entry block (#…

…8206)

Issue: fix #8203 

### Brief Summary

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### <samp>🤖 Generated by Copilot at 4f313de</samp>

Refactor the LLVM backend to use a better memory allocation strategy
with `create_entry_block_alloca`. This improves performance and
correctness of the generated code.

### Walkthrough

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copilot:walkthrough
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### <samp>🤖 Generated by Copilot at 4f313de</samp>

* Use `create_entry_block_alloca` function to allocate memory for LLVM
variables
([link](https://github.com/taichi-dev/taichi/pull/8206/files?diff=unified&w=0#diff-3c663c78745adcd3f6a7ac81fe99e628decc3040f292ea1e20ecd4b85a7f4313L2784-R2788),
[link](https://github.com/taichi-dev/taichi/pull/8206/files?diff=unified&w=0#diff-3c663c78745adcd3f6a7ac81fe99e628decc3040f292ea1e20ecd4b85a7f4313L2795-R2795)).
This avoids potential stack size and alignment issues in the LLVM
backend. The variables are `new_ctx`, `buffer`, and `result_buffer` in
`codegen_llvm.cpp`.

taichi/codegen/llvm/codegen_llvm.cpp

@@ -2781,18 +2781,18 @@ void TaskCodeGenLLVM::visit(FuncCallStmt *stmt) {
     current_callable = old_callable;
   }
   llvm::Function *llvm_func = func_map[stmt->func];
-  auto *new_ctx = builder->CreateAlloca(get_runtime_type("RuntimeContext"));
+  auto *new_ctx = create_entry_block_alloca(get_runtime_type("RuntimeContext"));
   call("RuntimeContext_set_runtime", new_ctx, get_runtime());
   if (!stmt->func->parameter_list.empty()) {
     auto *buffer =
-        builder->CreateAlloca(tlctx->get_data_type(stmt->func->args_type));
+        create_entry_block_alloca(tlctx->get_data_type(stmt->func->args_type));
     set_args_ptr(stmt->func, new_ctx, buffer);
     set_struct_to_buffer(stmt->func->args_type, buffer, stmt->args);
   }
   llvm::Value *result_buffer = nullptr;
   if (!stmt->func->rets.empty()) {
     auto *ret_type = tlctx->get_data_type(stmt->func->ret_type);
-    result_buffer = builder->CreateAlloca(ret_type);
+    result_buffer = create_entry_block_alloca(ret_type);
     auto *result_buffer_u64 = builder->CreatePointerCast(
         result_buffer,
         llvm::PointerType::get(tlctx->get_data_type<uint64>(), 0));