stage2: std.mem.eql works now

* The `indexable_ptr_len` ZIR instruction now uses a `none_or_ref`
   ResultLoc. This prevents an unnecessary `ref` instruction from being
   emitted.
 * Sema: Fix `analyzeCall` using the incorrect ZIR object for the
   generic function callee.
 * LLVM backend: `genTypedValue` supports a `Slice` type encoded with
   the `decl_ref` `Value`.

test/behavior/basic.zig

@@ -1,4 +1,5 @@
 const std = @import("std");
+const mem = std.mem;
 const expect = std.testing.expect;
 
 // normal comment
@@ -83,3 +84,11 @@ test "unicode escape in character literal" {
 test "unicode character in character literal" {
     try expect('💩' == 128169);
 }
+
+fn first4KeysOfHomeRow() []const u8 {
+    return "aoeu";
+}
+
+test "return string from function" {
+    try expect(mem.eql(u8, first4KeysOfHomeRow(), "aoeu"));
+}