Rollup merge of #150581 - Zalathar:string, r=Nadrieril

mir_build: Separate match lowering for string-equality and scalar-equality

- Follow-up to rust-lang/rust#150238
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This PR takes some match-lowering code that is responsible for equality tests, and splits it into distinct code paths for string-equality and scalar-equality.

The split results in more lines of code overall, but makes the separated code paths easier to understand individually.

r? Nadrieril

Author: JonathanBrouwer