[ir] Update get_rvalue_dtype and move it to class Expr

[lin-hitonami]

Stack from ghstack (oldest at bottom):

• [ir] [refactor] Let the type of Alloca and other statements be pointer #8007
• [ir] Update ASTBuilder for the refactor of Alloca #8126
• [ir] [refactor] Update passes for the refactor of Alloca #8125
• [ir] Update the codegen for the refactor of Alloca #8124
• [Error] Let to_primitive_type throw an error if the type is a pointer #8123
• [autodiff] Update autodiff for the refactor of Alloca #8122
• [refactor] [ir] Use get_rvalue_type in frontend IR instead of ret_type #8115
• [lang] Use get_rvalue_type instead of get_ret_type in python #8108
• -> [ir] Update get_rvalue_dtype and move it to class Expr #8107