cmd/compile: avoid panic in ternary rewrite on checked instructions

The replace function in rewritetern.go was panicing when encountering
instructions that had already been processed. This adds a check to
ensure we don't trigger a panic on these instructions.

Fixes #77582

Change-Id: I0b38312109b9cedaa1cb1320015097d62588a2fe
Reviewed-on: https://go-review.googlesource.com/c/go/+/745460
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>

src/cmd/compile/internal/ssa/issue77582_test.go

@@ -0,0 +1,29 @@
+// Copyright 2026 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.
+
+//go:build goexperiment.simd && amd64
+
+package ssa
+
+import (
+	"testing"
+
+	"cmd/compile/internal/archsimd"
+)
+
+func TestVPTERNLOGDPanic(t *testing.T) {
+	if !archsimd.X86.AVX512() {
+		t.Skip("Test only applies to AVX512")
+	}
+
+	resultsMask := archsimd.Mask64x8{}
+	a := archsimd.Mask64x8FromBits(0xFF)
+	b := archsimd.Float64x8{}.Less(archsimd.Float64x8{})
+
+	for i := 0; i < 1; i++ {
+		resultsMask = a.Or(b).Or(resultsMask)
+		// This nested logic triggered the panic
+		_ = resultsMask.And(resultsMask.And(archsimd.Mask64x8{}))
+	}
+}

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

@@ -174,6 +174,9 @@ func rewriteTern(f *Func) {
 	replace := func(a0 *Value, vars0 [3]*Value) {
 		imm := computeTT(a0, vars0)
 		op := ternOpForLogical(a0.Op)
+		if a0.Op >= OpAMD64LoweredGetClosurePtr {
+			return // It is already an AMD64 machine instruction
+		}
 		if op == a0.Op {
 			panic(fmt.Errorf("should have mapped away from input op, a0 is %s", a0.LongString()))
 		}