Support frontend type check for If/While statements

[strongoier]

Concisely describe the proposed feature
We have the following test:

taichi/tests/python/test_type_check.py

Lines 30 to 41 in 21d9048

	# @ti.test(arch=ti.cpu)
	# def test_ternary_op():
	# @ti.kernel
	# def select():
	# a = 1.1
	# b = 3
	# c = 3.6
	# d = b if a else c
	#
	# with pytest.raises(TypeError,
	# match="for 'select': 'f32', 'i32' and 'f32'"):
	# select()

, which was commented in #3441. This was because IfExp was expanded to regular If statements in our new AST builder in #3299, and we didn't have type checks for the conditions of If/While statements.

To make the type error messages for If/While statements as easy to read as normal expressions, and to uncomment the above test, we'd better support frontend type check for If/While statements.

[re-xyr]

Do we also need to typecheck other statements such as FrontendSNodeOpStmt or FrontendForStmt?

[re-xyr]

I found that Chi IR also has a typechecking pass in transforms/type_check.cpp. Does this mean after adding typechecking for frontend IR, we'll have two sets of typechecking mechanisms? This looks kind of redundant.

[strongoier]

Currently transforms/type_check.cpp does the following:

1. Check type errors;
2. Add implicit casting;
3. Fix some wrong types set by other passes.

However, we need to clearly separate user errors from compiler internal errors. Otherwise, neither users nor compiler developers can easily diagnose problems they meet.

Currently we are moving 1 to the frontend, and we just align the behavior with transforms/type_check.cpp for now. For example, begin/end of FrontendForStmt will be automatically cast, so we don't force them to be integers.

I believe in the next step we also need to move 2 to the frontend. And for 3, we need to fix our compiler implementation and finally turn this pass into an optional pure verification pass.