Rollup merge of #154879 - Zalathar:pattern-ty, r=Nadrieril

Don't store `pattern_ty` in `TestableCase`

This field's only remaining use was in an assertion, but we can perform the same assertion earlier when constructing `TestableCase::Range`.

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For background, the `pattern_ty` field was introduced in rust-lang/rust#136435 to replace a reference to the full THIR pattern node. Since then, most uses of `pattern_ty` were removed by rust-lang/rust#150238.

Author: JonathanBrouwer