internal/impl: CompressGZIP: simplify

This simplifies the code and fixes the following linter warning
reported by staticcheck:

        internal/impl/legacy_export.go:84:51: uint16(blockSize) ^ 0x0000 always equals uint16(blockSize) (SA4016)

Similar code:
 https://cs.opensource.google/go/go/+/master:src/compress/flate/huffman_bit_writer.go;l=413-414;drc=19309779ac5e2f5a2fd3cbb34421dafb2855ac21

Change-Id: Id099b0de635f668d44f57bacd89b564337560254
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/406841
Reviewed-by: Cassondra Foesch <cfoesch@gmail.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
diff --git a/internal/impl/legacy_export.go b/internal/impl/legacy_export.go
index e3fb0b5..d007e8c 100644
--- a/internal/impl/legacy_export.go
+++ b/internal/impl/legacy_export.go
@@ -81,8 +81,8 @@
 			blockHeader[0] = 0x01 // final bit per RFC 1951, section 3.2.3.
 			blockSize = len(in)
 		}
-		binary.LittleEndian.PutUint16(blockHeader[1:3], uint16(blockSize)^0x0000)
-		binary.LittleEndian.PutUint16(blockHeader[3:5], uint16(blockSize)^0xffff)
+		binary.LittleEndian.PutUint16(blockHeader[1:3], uint16(blockSize))
+		binary.LittleEndian.PutUint16(blockHeader[3:5], ^uint16(blockSize))
 		out = append(out, blockHeader[:]...)
 		out = append(out, in[:blockSize]...)
 		in = in[blockSize:]