Added a storage usage inverse order test

Author: ohadravid

tests/mir-opt/copy-prop/copy_prop_storage_preserve_head.g.CopyProp.diff

@@ -0,0 +1,23 @@
+- // MIR for `g` before CopyProp
++ // MIR for `g` after CopyProp
+  
+  fn g() -> usize {
+      let mut _0: usize;
+      let mut _1: usize;
+      let mut _2: usize;
+      let mut _3: usize;
+  
+      bb0: {
+-         StorageLive(_2);
+          StorageLive(_1);
+          _1 = const 0_usize;
+-         _2 = copy _1;
+-         _3 = copy _2;
+-         _0 = Add(copy _3, copy _3);
++         _0 = Add(copy _1, copy _1);
+          StorageDead(_1);
+-         StorageDead(_2);
+          return;
+      }
+  }
+  

tests/mir-opt/copy-prop/copy_prop_storage_preserve_head.rs

@@ -5,6 +5,7 @@
 use std::intrinsics::mir::*;
 
 // EMIT_MIR copy_prop_storage_preserve_head.f.CopyProp.diff
+// EMIT_MIR copy_prop_storage_preserve_head.g.CopyProp.diff
 
 #[custom_mir(dialect = "runtime", phase = "post-cleanup")]
 pub fn f(_1: &mut usize) {
@@ -27,3 +28,29 @@ pub fn f(_1: &mut usize) {
         }
     }
 }
+
+#[custom_mir(dialect = "runtime")]
+pub fn g() -> usize {
+    // CHECK-LABEL: fn g(
+    mir! {
+        let _1: usize;
+        let _2: usize;
+        let _3: usize;
+        // CHECK: bb0: {
+        {
+            // CHECK: StorageLive(_1);
+            // CHECK: _0 = Add(copy _1, copy _1);
+            // CHECK: StorageDead(_1);
+            StorageLive(_2);
+            StorageLive(_1);
+            _1 = 0;
+            _2 = _1;
+            _3 = _2;
+            RET = _3 + _3;
+            // Even though the storage statements are in reverse order, we should be able to keep the ones for _1.
+            StorageDead(_1);
+            StorageDead(_2);
+            Return()
+        }
+    }
+}