compress/flate: move big non-pointer arrays to end of compressor

compress/flate: move big non-pointer arrays to end of compressor

The compressor type is fairly large: 656616 bytes on amd64.
Before this patch, it had fields of slice and interface type
near the end of the struct. As those types always contain pointers,
the ptrBytes value in the type descriptor was quite large.
That forces the garbage collector to do extra work scanning for pointers,
and wastes a bit of executable space recording the gcmask for the type.

This patch moves the arrays to the end of the type,
fixing those minor issues.

Change-Id: I849a75a19cc61137c8797a1ea5a4c97e0f69b4db
Reviewed-on: https://go-review.googlesource.com/c/go/+/707596
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>