[ir][refactor] First step to move Frontend IR to its own file

[k-ye]

In this PR, I just moved FrontendAllocaStmt to a new file frontend_ir.h. If this looks OK, I will move all the other Frontend*Stmt (maybe *Expression as well ?)

Related issue = #689

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[yuanming-hu]

Looks good, thanks! And yes, let's move the Expressions as well. The expressions are only useful in the frontend before lower_ast. Not sure if we should create a separate file for the Expressions, but you should clearly have better judgment than I do! :-)

[k-ye]

Not sure if we should create a separate file for the Expressions

Cool! Not sure either, but I don't see an immediate need for separate Frontend Stmt and Expressions, so I will go with one file...

[yuanming-hu]

Sounds good! Given the current code structure, I don't think separating Exprs and Stmts is an easy job anyway...

[k-ye]

Will create another PR for the rest of the works..

[xumingkuan]

Thanks!

[xumingkuan]

Not sure if this is necessary after we move all Frontend*Stmts to their own file.

[k-ye]

I guess they have to be around for a while, since currently there are some legacy stuff referring to the Frontend stmts, e.g.

taichi/taichi/ir/ir.cpp

Lines 933 to 941 in 4d228b9

	For::For(const ExprGroup &i,
	const Expr &global,
	const std::function<void()> &func) {
	auto stmt_unique = std::make_unique<FrontendForStmt>(i, global);
	auto stmt = stmt_unique.get();
	current_ast_builder().insert(std::move(stmt_unique));
	auto _ = current_ast_builder().create_scope(stmt->body);
	func();
	}

I think once these legacy constructs are removed, then it should be fine to remove this dependency..