Add a test for a == b where a: !, b: !

(this currently produces malformed mir: we call `eq` with first argument not
being a reference)

Author: WaffleLapkin

tests/mir-opt/building/eq_never_type._f.built.after.mir

@@ -0,0 +1,53 @@
+// MIR for `_f` after built
+
+fn _f(_1: !, _2: !) -> () {
+    debug a => _1;
+    debug b => _2;
+    let mut _0: ();
+    let mut _3: !;
+    let _4: bool;
+    let mut _5: ();
+    let mut _6: !;
+    let mut _7: &();
+    let _8: ();
+    let mut _9: !;
+
+    bb0: {
+        StorageLive(_4);
+        StorageLive(_5);
+        StorageLive(_6);
+        _6 = _1;
+        unreachable;
+    }
+
+    bb1: {
+        StorageDead(_6);
+        StorageLive(_7);
+        StorageLive(_8);
+        StorageLive(_9);
+        _9 = _2;
+        unreachable;
+    }
+
+    bb2: {
+        _7 = &_8;
+        StorageDead(_9);
+        _4 = <() as PartialEq>::eq(move _5, move _7) -> [return: bb3, unwind: bb5];
+    }
+
+    bb3: {
+        StorageDead(_7);
+        StorageDead(_5);
+        StorageDead(_8);
+        StorageDead(_4);
+        unreachable;
+    }
+
+    bb4: {
+        return;
+    }
+
+    bb5 (cleanup): {
+        resume;
+    }
+}

tests/mir-opt/building/eq_never_type.rs

@@ -0,0 +1,13 @@
+// skip-filecheck
+#![feature(never_type)]
+#![allow(unreachable_code)]
+
+// EMIT_MIR eq_never_type._f.built.after.mir
+fn _f(a: !, b: !) {
+    // Both arguments must be references (i.e. == should auto-borrow/coerce-to-ref both arguments)
+    // (this previously was buggy due to `NeverToAny` coercion incorrectly throwing out other
+    // coercions)
+    a == b;
+}
+
+fn main() {}