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

Author: JakobDegen

src/test/mir-opt/early_otherwise_branch_68867.rs

@@ -1,4 +1,6 @@
-// compile-flags: -Z mir-opt-level=4 -Zunsound-mir-opts
+// unit-test: EarlyOtherwiseBranch
+
+// FIXME: This test was broken by the derefer change.
 
 // example from #68867
 type CSSFloat = f32;
@@ -11,7 +13,6 @@ pub enum ViewportPercentageLength {
 }
 
 // EMIT_MIR early_otherwise_branch_68867.try_sum.EarlyOtherwiseBranch.diff
-// EMIT_MIR early_otherwise_branch_68867.try_sum EarlyOtherwiseBranch.before SimplifyConstCondition-final.after
 #[no_mangle]
 pub extern "C" fn try_sum(
     x: &ViewportPercentageLength,