Demonstrate the bug

tests/mir-opt/instcombine_duplicate_switch_targets_e2e.rs

@@ -0,0 +1,16 @@
+// compile-flags: -Zmir-opt-level=2 -Zinline-mir
+// ignore-debug: standard library debug assertions add a panic that breaks this optimization
+#![crate_type = "lib"]
+
+pub enum Thing {
+    A,
+    B,
+}
+
+// EMIT_MIR instcombine_duplicate_switch_targets_e2e.ub_if_b.PreCodegen.after.mir
+pub unsafe fn ub_if_b(t: Thing) -> Thing {
+    match t {
+        Thing::A => t,
+        Thing::B => std::hint::unreachable_unchecked(),
+    }
+}

tests/mir-opt/instcombine_duplicate_switch_targets_e2e.ub_if_b.PreCodegen.after.mir

@@ -0,0 +1,27 @@
+// MIR for `ub_if_b` after PreCodegen
+
+fn ub_if_b(_1: Thing) -> Thing {
+    debug t => _1;                       // in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+0:23: +0:24
+    let mut _0: Thing;                   // return place in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+0:36: +0:41
+    let mut _2: isize;                   // in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+2:9: +2:17
+    scope 1 (inlined unreachable_unchecked) { // at $DIR/instcombine_duplicate_switch_targets_e2e.rs:14:21: 14:55
+        scope 2 {
+            scope 3 (inlined unreachable_unchecked::runtime) { // at $SRC_DIR/core/src/intrinsics.rs:LL:COL
+            }
+        }
+    }
+
+    bb0: {
+        _2 = discriminant(_1);           // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+1:11: +1:12
+        switchInt(move _2) -> [0: bb2, 1: bb1, otherwise: bb1]; // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+1:5: +1:12
+    }
+
+    bb1: {
+        unreachable;                     // scope 2 at $SRC_DIR/core/src/intrinsics.rs:LL:COL
+    }
+
+    bb2: {
+        _0 = move _1;                    // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+2:21: +2:22
+        return;                          // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+5:2: +5:2
+    }
+}