compress/flate: take NewWriter out of the benchmark loop.

compress/flate: take NewWriter out of the benchmark loop.

This helps follow-up CLs ensure that the encoding's core computation does not
allocate. It is a separate CL because it has a non-trivial effect on the
benchmark numbers, even if it's purely an accounting change and not a change to
the underlying performance:

BenchmarkEncodeDigitsSpeed1e4-4        5.65         19.31        3.42x
BenchmarkEncodeDigitsSpeed1e5-4        17.23        26.79        1.55x
BenchmarkEncodeDigitsSpeed1e6-4        26.85        27.51        1.02x
BenchmarkEncodeDigitsDefault1e4-4      4.41         13.21        3.00x
BenchmarkEncodeDigitsDefault1e5-4      5.64         6.28         1.11x
BenchmarkEncodeDigitsDefault1e6-4      5.54         5.65         1.02x
BenchmarkEncodeDigitsCompress1e4-4     4.31         13.15        3.05x
BenchmarkEncodeDigitsCompress1e5-4     5.52         5.91         1.07x
BenchmarkEncodeDigitsCompress1e6-4     5.38         5.63         1.05x
BenchmarkEncodeTwainSpeed1e4-4         5.45         19.06        3.50x
BenchmarkEncodeTwainSpeed1e5-4         17.30        29.25        1.69x
BenchmarkEncodeTwainSpeed1e6-4         28.06        30.86        1.10x
BenchmarkEncodeTwainDefault1e4-4       4.06         12.36        3.04x
BenchmarkEncodeTwainDefault1e5-4       6.15         7.62         1.24x
BenchmarkEncodeTwainDefault1e6-4       6.84         6.99         1.02x
BenchmarkEncodeTwainCompress1e4-4      4.06         12.27        3.02x
BenchmarkEncodeTwainCompress1e5-4      5.29         5.92         1.12x
BenchmarkEncodeTwainCompress1e6-4      5.24         5.29         1.01x

Change-Id: I7d32866b7e2d478b0154332c1edeefe339af9a28
Reviewed-on: https://go-review.googlesource.com/20467
Reviewed-by: David Symonds <dsymonds@golang.org>