regexp: reduce mallocs in Regexp.Find* and Regexp.ReplaceAll*.

regexp: reduce mallocs in Regexp.Find* and Regexp.ReplaceAll*.

This improves Regexp.Find* and Regexp.ReplaceAll* speed:

name                  old time/op    new time/op    delta
Find-4                   345ns ± 1%     314ns ± 1%    -8.94%    (p=0.000 n=9+8)
FindString-4             341ns ± 1%     308ns ± 0%    -9.85%   (p=0.000 n=10+9)
FindSubmatch-4           440ns ± 1%     404ns ± 0%    -8.27%   (p=0.000 n=10+8)
FindStringSubmatch-4     426ns ± 0%     387ns ± 0%    -9.07%   (p=0.000 n=10+9)
ReplaceAll-4            1.75µs ± 1%    1.67µs ± 0%    -4.45%   (p=0.000 n=9+10)

name                  old alloc/op   new alloc/op   delta
Find-4                   16.0B ± 0%     0.0B ±NaN%  -100.00%  (p=0.000 n=10+10)
FindString-4             16.0B ± 0%     0.0B ±NaN%  -100.00%  (p=0.000 n=10+10)
FindSubmatch-4           80.0B ± 0%     48.0B ± 0%   -40.00%  (p=0.000 n=10+10)
FindStringSubmatch-4     64.0B ± 0%     32.0B ± 0%   -50.00%  (p=0.000 n=10+10)
ReplaceAll-4              152B ± 0%      104B ± 0%   -31.58%  (p=0.000 n=10+10)

name                  old allocs/op  new allocs/op  delta
Find-4                    1.00 ± 0%     0.00 ±NaN%  -100.00%  (p=0.000 n=10+10)
FindString-4              1.00 ± 0%     0.00 ±NaN%  -100.00%  (p=0.000 n=10+10)
FindSubmatch-4            2.00 ± 0%      1.00 ± 0%   -50.00%  (p=0.000 n=10+10)
FindStringSubmatch-4      2.00 ± 0%      1.00 ± 0%   -50.00%  (p=0.000 n=10+10)
ReplaceAll-4              8.00 ± 0%      5.00 ± 0%   -37.50%  (p=0.000 n=10+10)

Fixes #15643

Change-Id: I594fe51172373e2adb98d1d25c76ca2cde54ff48
Reviewed-on: https://go-review.googlesource.com/23030
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>