Add FileCheck annotations to 5 mir-opt tests

Remove skip-filecheck and add FileCheck directives for:
- multiple_return_terminators.rs: verify pass collapses to single bb0 with return
- impossible_predicates.rs: verify body becomes unreachable when predicates impossible
- byte_slice.rs: verify byte slice literal lowering in MIR
- lower_array_len.rs: verify array .len() lowered to const N by GVN
- remove_zsts.rs: verify zero-sized types replaced with const ZeroSized

Author: sivadeilra

tests/mir-opt/byte_slice.rs

@@ -1,8 +1,10 @@
-// skip-filecheck
 //@ compile-flags: -Z mir-opt-level=0
 
 // EMIT_MIR byte_slice.main.SimplifyCfg-pre-optimizations.after.mir
 fn main() {
+    // CHECK-LABEL: fn main(
+    // CHECK: = const b"foo"
+    // CHECK: = [const 5_u8, const 120_u8]
     let x = b"foo";
     let y = [5u8, b'x'];
 }

tests/mir-opt/derefer_inline_test.rs

@@ -1,4 +1,3 @@
-// skip-filecheck
 //@ test-mir-pass: Derefer
 // EMIT_MIR derefer_inline_test.main.Derefer.diff
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
@@ -8,5 +7,7 @@ fn f() -> Box<u32> {
     Box::new(0)
 }
 fn main() {
+    // CHECK-LABEL: fn main(
+    // CHECK-NOT: deref_copy
     Box::new(f());
 }

tests/mir-opt/impossible_predicates.rs

@@ -1,10 +1,13 @@
-// skip-filecheck
 // EMIT_MIR impossible_predicates.impossible_predicate.ImpossiblePredicates.diff
 
 pub fn impossible_predicate(x: &mut i32) -> (&mut i32, &mut i32)
 where
     for<'a> &'a mut i32: Copy,
 {
+    // CHECK-LABEL: fn impossible_predicate(
+    // CHECK: bb0: {
+    // CHECK-NEXT: unreachable;
+    // CHECK-NEXT: }
     let y = x;
     (y, x)
 }

tests/mir-opt/issue_78192.rs

@@ -1,6 +1,7 @@
-// skip-filecheck
 //@ compile-flags: -Zmir-opt-level=1 -Zinline-mir
 pub fn f<T>(a: &T) -> *const T {
+    // CHECK-LABEL: fn f(
+    // CHECK: &raw const (*_1)
     let b: &*const T = &(a as *const T);
     *b
 }

tests/mir-opt/multiple_return_terminators.rs

@@ -1,8 +1,11 @@
-// skip-filecheck
 //@ compile-flags: -Z mir-opt-level=4
 // EMIT_MIR multiple_return_terminators.test.MultipleReturnTerminators.diff
 
 fn test(x: bool) {
+    // CHECK-LABEL: fn test(
+    // CHECK: bb0: {
+    // CHECK-NEXT: return;
+    // CHECK-NEXT: }
     if x {
         // test
     } else {