Explain the reason for why a test exists

tests/codegen/const_scalar_pair.rs

@@ -2,6 +2,8 @@
 
 #![feature(inline_const)]
 
+// Test that we don't generate a memory allocation for the constant
+// and read the fields from that, but instead just create the value pair directly.
 pub fn foo() -> (i32, i32) {
     // CHECK: ret { i32, i32 } { i32 1, i32 2 }
     const { (1, 2) }