Rollup merge of #100827 - JakobDegen:better-tests, r=wesleywiser

Rollup merge of #100827 - JakobDegen:better-tests, r=wesleywiser

Simplify MIR opt tests

This commit removes many cases of MIR opt tests emitting `.diff`s for more than one pass. These tests cannot be `unit-test`s, and so they are easy to break, and they also provide little value due to having excessively strong opinions over *how* a piece of code should be optimized.

Where reasonable, we instead add separate test files that only emit the `PreCodegen.after` MIR for code where we want to track what the end to end effect of the optimization pipeline on the example code is.

r? `````@wesleywiser`````