encoding/base64, encoding/base32: speed up Encode

Avoid unnecessary bitwise-OR operations.

benchmark                      old MB/s     new MB/s  speedup
BenchmarkEncodeToStringBase64  179.02       205.74    1.15x
BenchmarkEncodeToStringBase32  155.86       167.82    1.08x

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/109090043

src/pkg/encoding/base32/base32.go

@@ -79,26 +79,26 @@ func (enc *Encoding) Encode(dst, src []byte) {
 		// destination quantum
 		switch len(src) {
 		default:
-			b7 |= src[4] & 0x1F
-			b6 |= src[4] >> 5
+			b7 = src[4] & 0x1F
+			b6 = src[4] >> 5
 			fallthrough
 		case 4:
 			b6 |= (src[3] << 3) & 0x1F
-			b5 |= (src[3] >> 2) & 0x1F
-			b4 |= src[3] >> 7
+			b5 = (src[3] >> 2) & 0x1F
+			b4 = src[3] >> 7
 			fallthrough
 		case 3:
 			b4 |= (src[2] << 1) & 0x1F
-			b3 |= (src[2] >> 4) & 0x1F
+			b3 = (src[2] >> 4) & 0x1F
 			fallthrough
 		case 2:
 			b3 |= (src[1] << 4) & 0x1F
-			b2 |= (src[1] >> 1) & 0x1F
-			b1 |= (src[1] >> 6) & 0x1F
+			b2 = (src[1] >> 1) & 0x1F
+			b1 = (src[1] >> 6) & 0x1F
 			fallthrough
 		case 1:
 			b1 |= (src[0] << 2) & 0x1F
-			b0 |= src[0] >> 3
+			b0 = src[0] >> 3
 		}
 
 		// Encode 5-bit blocks using the base32 alphabet

src/pkg/encoding/base64/base64.go

@@ -80,16 +80,16 @@ func (enc *Encoding) Encode(dst, src []byte) {
 		// destination quantum
 		switch len(src) {
 		default:
-			b3 |= src[2] & 0x3F
-			b2 |= src[2] >> 6
+			b3 = src[2] & 0x3F
+			b2 = src[2] >> 6
 			fallthrough
 		case 2:
 			b2 |= (src[1] << 2) & 0x3F
-			b1 |= src[1] >> 4
+			b1 = src[1] >> 4
 			fallthrough
 		case 1:
 			b1 |= (src[0] << 4) & 0x3F
-			b0 |= src[0] >> 2
+			b0 = src[0] >> 2
 		}
 
 		// Encode 6-bit blocks using the base64 alphabet