compress/flate: tweak offsetCode so that it can be inlined

compress/flate: tweak offsetCode so that it can be inlined

Functions with switches (#13071) cannot be inlined.
Functions with consts (#7655) cannot be inlined.

benchmark                              old MB/s     new MB/s     speedup
BenchmarkEncodeDigitsSpeed1e4-4        10.25        10.20        1.00x
BenchmarkEncodeDigitsSpeed1e5-4        26.44        27.22        1.03x
BenchmarkEncodeDigitsSpeed1e6-4        32.28        33.51        1.04x
BenchmarkEncodeDigitsDefault1e4-4      8.61         8.74         1.02x
BenchmarkEncodeDigitsDefault1e5-4      7.03         6.98         0.99x
BenchmarkEncodeDigitsDefault1e6-4      6.47         6.46         1.00x
BenchmarkEncodeDigitsCompress1e4-4     8.62         8.73         1.01x
BenchmarkEncodeDigitsCompress1e5-4     7.01         6.98         1.00x
BenchmarkEncodeDigitsCompress1e6-4     6.43         6.53         1.02x
BenchmarkEncodeTwainSpeed1e4-4         9.67         10.16        1.05x
BenchmarkEncodeTwainSpeed1e5-4         26.46        26.94        1.02x
BenchmarkEncodeTwainSpeed1e6-4         33.19        34.02        1.03x
BenchmarkEncodeTwainDefault1e4-4       8.12         8.37         1.03x
BenchmarkEncodeTwainDefault1e5-4       8.22         8.21         1.00x
BenchmarkEncodeTwainDefault1e6-4       8.10         8.13         1.00x
BenchmarkEncodeTwainCompress1e4-4      8.24         8.39         1.02x
BenchmarkEncodeTwainCompress1e5-4      6.51         6.58         1.01x
BenchmarkEncodeTwainCompress1e6-4      6.16         6.13         1.00x

Change-Id: Ibafa5e3e2de0529853b5b3180e6fd6cb7090b76f
Reviewed-on: https://go-review.googlesource.com/17171
Reviewed-by: Russ Cox <rsc@golang.org>