Add FileCheck annotations to mir-opt/issues

From #123067, this test should ensure that there is no duplicated instruction block against or-patterns.

Signed-off-by: Shunpoco <[email protected]>

tests/mir-opt/issues/issue_75439.rs

@@ -1,10 +1,13 @@
-// skip-filecheck
 // EMIT_MIR issue_75439.foo.MatchBranchSimplification.diff
 //@ ignore-endian-big
 
 use std::mem::transmute;
 
 pub fn foo(bytes: [u8; 16]) -> Option<[u8; 4]> {
+    // CHECK-LABEL: fn foo(
+    // CHECK: bb2: {
+    // CHECK-NEXT: 0: bb3
+    // CHECK-SAME: 4294901760: bb3
     // big endian `u32`s
     let dwords: [u32; 4] = unsafe { transmute(bytes) };
     const FF: u32 = 0x0000_ffff_u32.to_be();