Add regression test for saturating_sub bounds check issue

Add codegen test for issue where `valid_index.saturating_sub(X)` produced an
extra bounds check.
This was fixed by the LLVM upgrade.

Author: okaneco

tests/codegen-llvm/issues/saturating-sub-index-139759.rs

@@ -0,0 +1,19 @@
+// Test that calculating an index with saturating subtraction from an in-bounds
+// index doesn't generate another bounds check.
+
+//@ compile-flags: -Copt-level=3
+//@ min-llvm-version: 21
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @bounds_check_is_elided
+#[no_mangle]
+pub fn bounds_check_is_elided(s: &[i32], index: usize) -> i32 {
+    // CHECK-NOT: panic_bounds_check
+    if index < s.len() {
+        let lower_bound = index.saturating_sub(1);
+        s[lower_bound]
+    } else {
+        -1
+    }
+}