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 result of the net result of the optimization pipeline's output
is.
