Add FileCheck to slice_len.rs

Author: sfzhu93

tests/mir-opt/dataflow-const-prop/slice_len.rs

@@ -1,13 +1,23 @@
-// skip-filecheck
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 // unit-test: DataflowConstProp
 // compile-flags: -Zmir-enable-passes=+InstSimplify
 // EMIT_MIR_FOR_EACH_BIT_WIDTH
 
 // EMIT_MIR slice_len.main.DataflowConstProp.diff
+
+// CHECK-LABEL: fn main
 fn main() {
+    // CHECK: debug local => [[local:_[0-9]+]];
+    // CHECK: debug constant => [[constant:_[0-9]+]];
+
+    // CHECK: {{_[0-9]+}} = const 3_usize;
+    // CHECK: {{_[0-9]+}} = const true;
+
+    // CHECK: [[local]] = (*{{_[0-9]+}})[1 of 2];
     let local = (&[1u32, 2, 3] as &[u32])[1];
 
     const SLICE: &[u32] = &[1, 2, 3];
+
+    // CHECK: [[constant]] = (*{{_[0-9]+}})[1 of 2];
     let constant = SLICE[1];
 }