chacha20poly1305: drop pre-AVX assembly impl

Updates golang/go#69587

Change-Id: Ic158558f879b8b8ad23155bf887e083169096d19
Reviewed-on: https://go-review.googlesource.com/c/crypto/+/672837
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>

chacha20poly1305/chacha20poly1305_amd64.go

@@ -20,7 +20,7 @@ func chacha20Poly1305Open(dst []byte, key []uint32, src, ad []byte) bool
 func chacha20Poly1305Seal(dst []byte, key []uint32, src, ad []byte)
 
 var (
-	useAVX2 = cpu.X86.HasAVX2 && cpu.X86.HasBMI2
+	useAVX2 = cpu.X86.HasSSSE3 && cpu.X86.HasAVX2 && cpu.X86.HasBMI2
 )
 
 // setupState writes a ChaCha20 input matrix to state. See
@@ -47,7 +47,7 @@ func setupState(state *[16]uint32, key *[32]byte, nonce []byte) {
 }
 
 func (c *chacha20poly1305) seal(dst, nonce, plaintext, additionalData []byte) []byte {
-	if !cpu.X86.HasSSSE3 {
+	if !useAVX2 {
 		return c.sealGeneric(dst, nonce, plaintext, additionalData)
 	}
 
@@ -66,7 +66,7 @@ func (c *chacha20poly1305) seal(dst, nonce, plaintext, additionalData []byte) []
 }
 
 func (c *chacha20poly1305) open(dst, nonce, ciphertext, additionalData []byte) ([]byte, error) {
-	if !cpu.X86.HasSSSE3 {
+	if !useAVX2 {
 		return c.openGeneric(dst, nonce, ciphertext, additionalData)
 	}