tests: mark option-niche-eq as fixed on LLVM 21

Some combination of recent Rust changes (between 3d86494 and
aa57e46 from what I can tell) and changes in LLVM 21 (not recently,
as best I can tell) have caused this test to start showing the behavior
we want, so it's time to move this test to a proper place and mark it as
fixed on LLVM 21.

Author: durin42

tests/codegen/option-niche-eq.rs

@@ -1,5 +1,7 @@
+//@ revisions: REGULAR LLVM21
 //@ min-llvm-version: 20
 //@ compile-flags: -Copt-level=3 -Zmerge-functions=disabled
+//@ [LLVM21] min-llvm-version: 21
 #![crate_type = "lib"]
 
 extern crate core;
@@ -74,3 +76,12 @@ pub fn niche_eq(l: Option<EnumWithNiche>, r: Option<EnumWithNiche>) -> bool {
     // CHECK-NEXT: ret i1
     l == r
 }
+
+// LLVM21-LABEL: @bool_eq
+#[no_mangle]
+pub fn bool_eq(l: Option<bool>, r: Option<bool>) -> bool {
+    // LLVM21: start:
+    // LLVM21-NEXT: icmp eq i8
+    // LLVM21-NEXT: ret i1
+    l == r
+}