Enable function merging when opt is for size

It is, of course, natural to want to merge aliasing functions when
optimizing for code size, since that can eliminate several bytes.
And an exhaustive match helps make the code less brittle.

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -255,8 +255,11 @@ macro_rules! if_regular {
             {
                 MergeFunctions::Disabled => false,
                 MergeFunctions::Trampolines | MergeFunctions::Aliases => {
-                    sess.opts.optimize == config::OptLevel::Default
-                        || sess.opts.optimize == config::OptLevel::Aggressive
+                    use config::OptLevel::*;
+                    match sess.opts.optimize {
+                        Aggressive | Default | SizeMin | Size => true,
+                        Less | No => false,
+                    }
                 }
             },
 

src/test/codegen/merge-functions.rs

@@ -1,7 +1,9 @@
-// compile-flags: -O
+// revisions: O Os
+//[Os] compile-flags: -Copt-level=s
+//[O] compile-flags: -O
 #![crate_type = "lib"]
 
-// CHECK: @func2 = {{.*}}alias{{.*}}@func1
+// CHECK: @func{{2|1}} = {{.*}}alias{{.*}}@func{{1|2}}
 
 #[no_mangle]
 pub fn func1(c: char) -> bool {