Add FileCheck to dead_stores_79191.rs

Author: Shunpoco

tests/mir-opt/copy-prop/dead_stores_79191.rs

@@ -1,4 +1,3 @@
-// skip-filecheck
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 //@ test-mir-pass: CopyProp
 
@@ -8,6 +7,14 @@ fn id<T>(x: T) -> T {
 
 // EMIT_MIR dead_stores_79191.f.CopyProp.after.mir
 fn f(mut a: usize) -> usize {
+    // CHECK-LABEL: fn f(
+    // CHECK: debug a => [[a:_.*]];
+    // CHECK: debug b => [[b:_.*]];
+    // CHECK: [[b]] = copy [[a]];
+    // CHECK: [[a]] = const 5_usize;
+    // CHECK: [[a]] = copy [[b]];
+    // CHECK: [[c:_.*]] = copy [[a]]
+    // CHECK: id::<usize>(move [[c]])
     let b = a;
     a = 5;
     a = b;