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.

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,

src/test/mir-opt/early_otherwise_branch_68867.try_sum.EarlyOtherwiseBranch.diff

@@ -80,7 +80,7 @@
           StorageDead(_5);                 // scope 0 at $DIR/early_otherwise_branch_68867.rs:+5:23: +5:24
           _34 = deref_copy (_4.0: &ViewportPercentageLength); // scope 0 at $DIR/early_otherwise_branch_68867.rs:+5:14: +5:24
           _11 = discriminant((*_34));      // scope 0 at $DIR/early_otherwise_branch_68867.rs:+5:14: +5:24
-          switchInt(move _11) -> [0_isize: bb1, 1_isize: bb3, 2_isize: bb4, 3_isize: bb5, otherwise: bb11]; // scope 0 at $DIR/early_otherwise_branch_68867.rs:+5:8: +5:24
+          switchInt(move _11) -> [0_isize: bb1, 1_isize: bb3, 2_isize: bb4, 3_isize: bb5, otherwise: bb2]; // scope 0 at $DIR/early_otherwise_branch_68867.rs:+5:8: +5:24
       }
 
       bb1: {
@@ -91,14 +91,14 @@
 
       bb2: {
           StorageLive(_33);                // scope 0 at $DIR/early_otherwise_branch_68867.rs:+10:25: +10:27
-          nop;                             // scope 0 at $DIR/early_otherwise_branch_68867.rs:+10:25: +10:27
+          Deinit(_33);                     // scope 0 at $DIR/early_otherwise_branch_68867.rs:+10:25: +10:27
           Deinit(_0);                      // scope 0 at $DIR/early_otherwise_branch_68867.rs:+10:21: +10:28
-          nop;                             // scope 0 at $DIR/early_otherwise_branch_68867.rs:+10:21: +10:28
+          ((_0 as Err).0: ()) = move _33;  // scope 0 at $DIR/early_otherwise_branch_68867.rs:+10:21: +10:28
           discriminant(_0) = 1;            // scope 0 at $DIR/early_otherwise_branch_68867.rs:+10:21: +10:28
           StorageDead(_33);                // scope 0 at $DIR/early_otherwise_branch_68867.rs:+10:27: +10:28
           StorageDead(_3);                 // scope 0 at $DIR/early_otherwise_branch_68867.rs:+11:6: +11:7
           StorageDead(_4);                 // scope 0 at $DIR/early_otherwise_branch_68867.rs:+12:1: +12:2
-          return;                          // scope 0 at $DIR/early_otherwise_branch_68867.rs:+12:2: +12:2
+          goto -> bb11;                    // scope 0 at $DIR/early_otherwise_branch_68867.rs:+12:2: +12:2
       }
 
       bb3: {
@@ -221,11 +221,11 @@
           discriminant(_0) = 0;            // scope 0 at $DIR/early_otherwise_branch_68867.rs:+5:5: +11:7
           StorageDead(_3);                 // scope 0 at $DIR/early_otherwise_branch_68867.rs:+11:6: +11:7
           StorageDead(_4);                 // scope 0 at $DIR/early_otherwise_branch_68867.rs:+12:1: +12:2
-          return;                          // scope 0 at $DIR/early_otherwise_branch_68867.rs:+12:2: +12:2
+          goto -> bb11;                    // scope 0 at $DIR/early_otherwise_branch_68867.rs:+12:2: +12:2
       }
 
       bb11: {
-          unreachable;                     // scope 0 at $DIR/early_otherwise_branch_68867.rs:+12:2: +12:2
+          return;                          // scope 0 at $DIR/early_otherwise_branch_68867.rs:+12:2: +12:2
       }
   }
 

src/test/mir-opt/issue-73223.rs

@@ -10,4 +10,3 @@ fn main() {
 
 // EMIT_MIR_FOR_EACH_BIT_WIDTH
 // EMIT_MIR issue_73223.main.SimplifyArmIdentity.diff
-// EMIT_MIR issue_73223.main.PreCodegen.diff