Reduce size of Method::parse_types (#7613)

* add maybe_unused where needed in size_test.cpp

At least one internal build have -Werror -Wunused-variable and ExecuTorch logging disabled.

Differential Revision: [D68032964](https://our.internmc.facebook.com/intern/diff/D68032964/)

ghstack-source-id: 260945746
Pull Request resolved: https://github.com/pytorch/executorch/pull/7602

* Reduce size of Method::parse_types

It looks like this function was big enough that inlining gave up, since I was seeing non-inlined calls to val_as_Foo() that should have been easily optimized away. Do said optimization manually.

Differential Revision: [D68037113](https://our.internmc.facebook.com/intern/diff/D68037113/)

ghstack-source-id: 260959235
Pull Request resolved: https://github.com/pytorch/executorch/pull/7603

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Co-authored-by: Github Executorch <github_executorch@arm.com>