Added test for preserving head storage

Author: ohadravid

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

@@ -0,0 +1,20 @@
+- // MIR for `f` before CopyProp
++ // MIR for `f` after CopyProp
+  
+  fn f(_1: &mut usize) -> () {
+      let mut _0: ();
+      let mut _2: usize;
+      let mut _3: usize;
+  
+      bb0: {
+          StorageLive(_2);
+          _2 = const 0_usize;
+-         _3 = copy _2;
+-         (*_1) = copy _3;
++         (*_1) = copy _2;
+          StorageDead(_2);
+          (*_1) = copy _2;
+          return;
+      }
+  }
+  

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

@@ -0,0 +1,25 @@
+// skip-filecheck
+//@ test-mir-pass: CopyProp
+//@ compile-flags: -Zlint-mir=false
+
+#![feature(custom_mir, core_intrinsics)]
+use std::intrinsics::mir::*;
+
+// EMIT_MIR copy_prop_storage_preserve_head.f.CopyProp.diff
+
+#[custom_mir(dialect = "runtime", phase = "post-cleanup")]
+pub fn f(_1: &mut usize) {
+    mir! {
+        let _2: usize;
+        let _3: usize;
+        {
+            StorageLive(_2);
+            _2 = 0;
+            _3 = _2;
+            (*_1) = _3;
+            StorageDead(_2);
+            (*_1) = _2;
+            Return()
+        }
+    }
+}