compress/flate: revert a goto for-loop

compress/flate: revert a goto for-loop

In https://golang.org/cl/16528, a goto loop was chosen over a regular
for loop since that would make the function inlinable.

Thanks to the recent https://golang.org/cl/256459, for loops without a
label can now be inlined. So we can undo the workaround and simplify the
code.

Also add the function to TestIntendedInlining, which passes both before
and after the change, as expected.

For #14768.

Change-Id: Ie5df55a6bcb07c538ca331eef2f908807ff0b516
Reviewed-on: https://go-review.googlesource.com/c/go/+/263037
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>