Add regression test showcasing miscompilation

src/test/mir-opt/76803_regression.encode.SimplifyBranchSame.diff

@@ -0,0 +1,30 @@
+- // MIR for `encode` before SimplifyBranchSame
++ // MIR for `encode` after SimplifyBranchSame
+  
+  fn encode(_1: Type) -> Type {
+      debug v => _1;                       // in scope 0 at $DIR/76803_regression.rs:10:15: 10:16
+      let mut _0: Type;                    // return place in scope 0 at $DIR/76803_regression.rs:10:27: 10:31
+      let mut _2: isize;                   // in scope 0 at $DIR/76803_regression.rs:12:9: 12:16
+  
+      bb0: {
+          _2 = discriminant(_1);           // scope 0 at $DIR/76803_regression.rs:12:9: 12:16
+-         switchInt(move _2) -> [0_isize: bb2, otherwise: bb1]; // scope 0 at $DIR/76803_regression.rs:12:9: 12:16
++         goto -> bb1;                     // scope 0 at $DIR/76803_regression.rs:12:9: 12:16
+      }
+  
+      bb1: {
+          _0 = move _1;                    // scope 0 at $DIR/76803_regression.rs:13:14: 13:15
+-         goto -> bb3;                     // scope 0 at $DIR/76803_regression.rs:11:5: 14:6
++         goto -> bb2;                     // scope 0 at $DIR/76803_regression.rs:11:5: 14:6
+      }
+  
+      bb2: {
+-         discriminant(_0) = 1;            // scope 0 at $DIR/76803_regression.rs:12:20: 12:27
+-         goto -> bb3;                     // scope 0 at $DIR/76803_regression.rs:11:5: 14:6
+-     }
+- 
+-     bb3: {
+          return;                          // scope 0 at $DIR/76803_regression.rs:15:2: 15:2
+      }
+  }
+  

src/test/mir-opt/76803_regression.rs

@@ -0,0 +1,19 @@
+// compile-flags: -Z mir-opt-level=1
+// EMIT_MIR 76803_regression.encode.SimplifyBranchSame.diff
+
+#[derive(Debug, Eq, PartialEq)]
+pub enum Type {
+    A,
+    B,
+}
+
+pub fn encode(v: Type) -> Type {
+    match v {
+        Type::A => Type::B,
+        _ => v,
+    }
+}
+
+fn main() {
+    assert_eq!(Type::B, encode(Type::A));
+}