Auto merge of #80475 - simonvandel:fix-77355, r=oli-obk

New mir-opt pass to simplify gotos with const values (reopening #77486)

Reopening PR #77486

Fixes #77355

This pass optimizes the following sequence
```rust
bb2: {
    _2 = const true;
    goto -> bb3;
}

bb3: {
    switchInt(_2) -> [false: bb4, otherwise: bb5];
}
```
into
```rust
bb2: {
    _2 = const true;
    goto -> bb5;
}
```

compiler/rustc_middle/src/mir/mod.rs

@@ -1518,7 +1518,14 @@ pub enum StatementKind<'tcx> {
 }
 
 impl<'tcx> StatementKind<'tcx> {
-    pub fn as_assign_mut(&mut self) -> Option<&mut Box<(Place<'tcx>, Rvalue<'tcx>)>> {
+    pub fn as_assign_mut(&mut self) -> Option<&mut (Place<'tcx>, Rvalue<'tcx>)> {
+        match self {
+            StatementKind::Assign(x) => Some(x),
+            _ => None,
+        }
+    }
+
+    pub fn as_assign(&self) -> Option<&(Place<'tcx>, Rvalue<'tcx>)> {
         match self {
             StatementKind::Assign(x) => Some(x),
             _ => None,

compiler/rustc_middle/src/mir/terminator.rs

@@ -407,6 +407,22 @@ impl<'tcx> TerminatorKind<'tcx> {
             | TerminatorKind::FalseUnwind { ref mut unwind, .. } => Some(unwind),
         }
     }
+
+    pub fn as_switch(&self) -> Option<(&Operand<'tcx>, Ty<'tcx>, &SwitchTargets)> {
+        match self {
+            TerminatorKind::SwitchInt { discr, switch_ty, targets } => {
+                Some((discr, switch_ty, targets))
+            }
+            _ => None,
+        }
+    }
+
+    pub fn as_goto(&self) -> Option<BasicBlock> {
+        match self {
+            TerminatorKind::Goto { target } => Some(*target),
+            _ => None,
+        }
+    }
 }
 
 impl<'tcx> Debug for TerminatorKind<'tcx> {