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

This reverts commit bcd25c79aa (CL 693415)

Reason for revert: still causing compiler failures on Google test code

Change-Id: I887edcff56bde3ffa316f2b629021ad323a357fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/694996
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

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 (IData x)) => (IData x)
+(StructSelect [0] (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    x) && x.Type.IsPtrShaped()  => x
+(StructSelect [0] x) && x.Type.IsPtrShaped()  => x
 (ArraySelect [0] x) && x.Type.IsPtrShaped()  => x
 
 // These, too.  Bits is bits.

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

@@ -839,10 +839,10 @@ func rewriteValuedec_OpStructMake(v *Value) bool {
 func rewriteValuedec_OpStructSelect(v *Value) bool {
 	v_0 := v.Args[0]
 	b := v.Block
-	// match: (StructSelect (IData x))
+	// match: (StructSelect [0] (IData x))
 	// result: (IData x)
 	for {
-		if v_0.Op != OpIData {
+		if auxIntToInt64(v.AuxInt) != 0 || v_0.Op != OpIData {
 			break
 		}
 		x := v_0.Args[0]
@@ -861,10 +861,13 @@ func rewriteValuedec_OpStructSelect(v *Value) bool {
 		v.copyOf(x.Args[i])
 		return true
 	}
-	// match: (StructSelect x)
+	// match: (StructSelect [0] x)
 	// cond: x.Type.IsPtrShaped()
 	// result: x
 	for {
+		if auxIntToInt64(v.AuxInt) != 0 {
+			break
+		}
 		x := v_0
 		if !(x.Type.IsPtrShaped()) {
 			break

test/fixedbugs/issue74888.go

@@ -1,20 +0,0 @@
-// 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) {
-}