Added a codegen test for optimization with const arrays

Closes #107208

tests/codegen/const-array.rs

@@ -0,0 +1,15 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+
+const LUT: [u8; 2] = [1, 1];
+
+// CHECK-LABEL: @decode
+#[no_mangle]
+pub fn decode(i: u8) -> u8 {
+    // CHECK: start:
+    // CHECK-NEXT: icmp
+    // CHECK-NEXT: select
+    // CHECK-NEXT: ret
+    if i < 2 { LUT[i as usize] } else { 2 }
+}