add a mir-opt test that we don't add the spurious drop

Author: nikomatsakis

src/test/mir-opt/no-spurious-drop-after-call.rs

@@ -0,0 +1,24 @@
+// ignore-wasm32-bare compiled with panic=abort by default
+
+// Test that after the call to `std::mem::drop` we do not generate a
+// MIR drop of the argument. (We used to have a `DROP(_2)` in the code
+// below, as part of bb3.)
+
+fn main() {
+    std::mem::drop("".to_string());
+}
+
+// END RUST SOURCE
+// START rustc.main.ElaborateDrops.before.mir
+//    bb2: {
+//        StorageDead(_3);
+//        _1 = const std::mem::drop::<std::string::String>(move _2) -> [return: bb3, unwind: bb4];
+//    }
+//    bb3: {
+//        StorageDead(_2);
+//        StorageDead(_4);
+//        StorageDead(_1);
+//        _0 = ();
+//        return;
+//    }
+// END rustc.main.ElaborateDrops.before.mir