[dev.simd] cmd/compile: track which CPU features are in scope

analysis for

- is this block only reached through feature checks?
- does the function signature imply AVX-something?
- is there an instruction in this block which implies AVX-something?

and keep track of which features those are.  Features =
AVX, AVX2, AVX512, etc.

Has a test.

Change-Id: I0b6f2e87d01ec587818db11cf71fac1e4d500650
Reviewed-on: https://go-review.googlesource.com/c/go/+/706337
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>

test/simd.go

@@ -0,0 +1,97 @@
+// errorcheck -0 -d=ssa/cpufeatures/debug=1
+
+//go:build goexperiment.simd && amd64
+
+// 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 foo
+
+import "simd"
+
+func f1(x simd.Int8x16) {
+	return // ERROR "has features avx"
+}
+
+func g1() simd.Int8x16 {
+	var x simd.Int8x16
+	return x // ERROR "has features avx$"
+}
+
+type T1 simd.Int8x16
+
+func (x T1) h() {
+	return // ERROR "has features avx$"
+}
+
+func f2(x simd.Int8x64) {
+	return // ERROR "has features avx[+]avx2[+]avx512$"
+}
+
+func g2() simd.Int8x64 {
+	var x simd.Int8x64
+	return x // ERROR "has features avx[+]avx2[+]avx512$"
+}
+
+type T2 simd.Int8x64
+
+func (x T2) h() {
+	return // ERROR "has features avx[+]avx2[+]avx512$"
+}
+
+var a int
+
+func f() {
+	if a == 0 {
+		if !simd.HasAVX512() {
+			return
+		}
+		println("has avx512") // ERROR "has features avx[+]avx2[+]avx512$"
+	} else {
+		if !simd.HasAVX2() {
+			return
+		}
+		println("has avx2") // ERROR "has features avx[+]avx2$"
+	}
+	println("has something")
+} // ERROR "has features avx[+]avx2$"
+
+func g() {
+	if simd.HasAVX2() { // ERROR "has features avx[+]avx2$"
+		for range 5 { // ERROR "has features avx[+]avx2$"
+			if a < 0 { // ERROR "has features avx[+]avx2$"
+				a++ // ERROR "has features avx[+]avx2$"
+			}
+		}
+	}
+	println("ahoy!") // ERROR "has features avx[+]avx2$" // this is an artifact of flaky block numbering and why isn't it fused?
+	if a > 0 {
+		a--
+	}
+}
+
+//go:noinline
+func p() bool {
+	return true
+}
+
+func hasIrreducibleLoop() {
+	if simd.HasAVX2() {
+		goto a // ERROR "has features avx[+]avx2$"
+	} else {
+		goto b
+	}
+a:
+	println("a")
+	if p() {
+		goto c
+	}
+b:
+	println("b")
+	if p() {
+		goto a
+	}
+c:
+	println("c")
+}