copy elision: double implicit cast

```zig
export fn entry() void {
    var x: error!?Foo = bar();
}
```

```llvm
define void @entry() #2 !dbg !41 {
Entry:
  %x = alloca { i16, { %Foo, i1 } }, align 4
  %0 = getelementptr inbounds { i16, { %Foo, i1 } }, { i16, { %Foo, i1 } }* %x, i32 0, i32 0, !dbg !61
  store i16 0, i16* %0, align 2, !dbg !61
  %1 = getelementptr inbounds { i16, { %Foo, i1 } }, { i16, { %Foo, i1 } }* %x, i32 0, i32 1, !dbg !61
  %2 = getelementptr inbounds { %Foo, i1 }, { %Foo, i1 }* %1, i32 0, i32 1, !dbg !61
  store i1 true, i1* %2, align 1, !dbg !61
  %3 = getelementptr inbounds { %Foo, i1 }, { %Foo, i1 }* %1, i32 0, i32 0, !dbg !61
  call fastcc void @bar(%Foo* sret %3), !dbg !62
  ret void, !dbg !63
}
```

Author: andrewrk

Diff for: src/ir.cpp

@@ -4935,9 +4935,8 @@ static IrInstruction *ir_gen_fn_call(IrBuilder *irb, Scope *scope, AstNode *node
     size_t arg_count = node->data.fn_call_expr.params.length;
     IrInstruction **args = allocate<IrInstruction*>(arg_count);
     for (size_t i = 0; i < arg_count; i += 1) {
-        IrInstruction *param_result_loc = ir_build_result_param(irb, scope, node, fn_ref, i);
         AstNode *arg_node = node->data.fn_call_expr.params.at(i);
-        args[i] = ir_gen_node(irb, arg_node, scope, LValNone, param_result_loc);
+        args[i] = ir_gen_node(irb, arg_node, scope, LValNone, nullptr);
         if (args[i] == irb->codegen->invalid_instruction)
             return args[i];
     }
@@ -10885,7 +10884,7 @@ static IrInstruction *ir_analyze_cast(IrAnalyze *ira, IrInstruction *source_inst
         }
     }
 
-    // cast from error set to error union type
+    // cast from E to E!T
     if (wanted_type->id == ZigTypeIdErrorUnion &&
         actual_type->id == ZigTypeIdErrorSet)
     {
@@ -11200,6 +11199,27 @@ static IrInstruction *ir_implicit_cast_result(IrAnalyze *ira, IrInstruction *res
         return ir_analyze_result_error_union_code(ira, result_loc, needed_child_type);
     }
 
+    // cast from T to E!?T
+    if (have_child_type->id == ZigTypeIdErrorUnion &&
+        have_child_type->data.error_union.payload_type->id == ZigTypeIdOptional &&
+        needed_child_type->id != ZigTypeIdOptional)
+    {
+        ZigType *wanted_child_type = have_child_type->data.error_union.payload_type->data.maybe.child_type;
+        if (types_match_const_cast_only(ira, wanted_child_type, needed_child_type, source_node,
+                false).id == ConstCastResultIdOk ||
+            needed_child_type->id == ZigTypeIdNull ||
+            needed_child_type->id == ZigTypeIdComptimeInt ||
+            needed_child_type->id == ZigTypeIdComptimeFloat)
+        {
+            IrInstruction *cast1 = ir_implicit_cast_result(ira, result_loc,
+                    have_child_type->data.error_union.payload_type);
+            if (type_is_invalid(cast1->value.type))
+                return ira->codegen->invalid_instruction;
+
+            return ir_implicit_cast_result(ira, cast1, needed_child_type);
+        }
+    }
+
     ErrorMsg *parent_msg = ir_add_error_node(ira, source_node,
         buf_sprintf("expected type '%s', found '%s'",
             buf_ptr(&have_child_type->name),