Add test for rust-lang#98294

Add a test to make that the failure condition for this pattern is
optimized away.

Fixes rust-lang#98294.

src/test/codegen/issue-98294-get-mut-copy-from-slice-opt.rs

@@ -0,0 +1,18 @@
+// min-llvm-version: 15.0.0
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+// There should be no calls to panic / len_mismatch_fail.
+
+#[no_mangle]
+pub fn test(a: &mut [u8], offset: usize, bytes: &[u8]) {
+    // CHECK-LABEL: @test(
+    // CHECK-NOT: call
+    // CHECK: call void @llvm.memcpy
+    // CHECK-NOT: call
+    // CHECK: }
+    if let Some(dst) = a.get_mut(offset..offset + bytes.len()) {
+        dst.copy_from_slice(bytes);
+    }
+}