Add peephold optimization that simplifies Ne(_1, false) and Ne(false, _1) into _1

This was observed emitted from the MatchBranchSimplification pass.

compiler/rustc_middle/src/mir/mod.rs

@@ -1954,6 +1954,15 @@ impl<'tcx> Operand<'tcx> {
             Operand::Constant(_) => None,
         }
     }
+
+    /// Returns the `Constant` that is the target of this `Operand`, or `None` if this `Operand` is a
+    /// place.
+    pub fn constant(&self) -> Option<&Constant<'tcx>> {
+        match self {
+            Operand::Constant(x) => Some(&**x),
+            Operand::Copy(_) | Operand::Move(_) => None,
+        }
+    }
 }
 
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