restructure unary_op to also dispatch on type first; fix promotion with unary '-' overflowing

src/test/run-pass/ctfe/promotion.rs

@@ -8,10 +8,18 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// compile-flags: -O
+
 fn foo(_: &'static [&'static str]) {}
 fn bar(_: &'static [&'static str; 3]) {}
+fn baz_i32(_: &'static i32) {}
+fn baz_u32(_: &'static u32) {}
 
 fn main() {
     foo(&["a", "b", "c"]);
     bar(&["d", "e", "f"]);
+
+    // make sure that these do not cause trouble despite overflowing
+    baz_u32(&(0-1));
+    baz_i32(&-std::i32::MIN);
 }