make noalias-box-off filecheck more precise

The CHECK, -NOT, -SAME pattern ensures that the `CHECK-NOT: noalias`
is limited to only one line, and won't match unrelated lines further
down in the file.

Explicit drop call added to preserve the `foo` argument name, since
names of unused arguments are not preserved.

Author: erikdesjardins

tests/codegen/noalias-box-off.rs

@@ -3,6 +3,9 @@
 #![crate_type = "lib"]
 
 // CHECK-LABEL: @box_should_not_have_noalias_if_disabled(
-// CHECK-NOT: noalias{{.*}}%
+// CHECK-NOT: noalias
+// CHECK-SAME: %foo)
 #[no_mangle]
-pub fn box_should_not_have_noalias_if_disabled(_b: Box<u8>) {}
+pub fn box_should_not_have_noalias_if_disabled(foo: Box<u8>) {
+    drop(foo);
+}