cmd/compile: allow StructSelect [x] of interface data fields for x>0

As of CL 681937 we can now have structs which are pointer shaped, but
their pointer field is not the first field, like struct{ struct{}; *int }.

Fixes #74888

Change-Id: Idc80f6b1abde3ae01437e2a9cadb5aa23d04b806
Reviewed-on: https://go-review.googlesource.com/c/go/+/693415
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: David Chase <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: randall77

src/cmd/compile/internal/ssa/_gen/dec.rules

@@ -98,7 +98,7 @@
 // Some of these are copied from generic.rules
 
 (IMake _typ (StructMake val)) => (IMake _typ val)
-(StructSelect [0] (IData x)) => (IData x)
+(StructSelect (IData x)) => (IData x)
 
 (StructSelect [i] x:(StructMake ___)) => x.Args[i]
 
@@ -109,7 +109,7 @@
 // More annoying case: (ArraySelect[0] (StructSelect[0] isAPtr))
 // There, result of the StructSelect is an Array (not a pointer) and
 // the pre-rewrite input to the ArraySelect is a struct, not a pointer.
-(StructSelect [0] x) && x.Type.IsPtrShaped()  => x
+(StructSelect    x) && x.Type.IsPtrShaped()  => x
 (ArraySelect [0] x) && x.Type.IsPtrShaped()  => x
 
 // These, too.  Bits is bits.

src/cmd/compile/internal/ssa/rewritedec.go

@@ -0,0 +1,20 @@
+// compile
+
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+type P struct {
+	q struct{}
+	p *int
+}
+
+func f(x any) {
+	h(x.(P))
+}
+
+//go:noinline
+func h(P) {
+}