Optimize.

Here's the improvement over the unoptimized version:
benchmark                         old ns/op     new ns/op     delta
BenchmarkRegexpMatchEasy0x32      16776         3496          -79.16%
BenchmarkRegexpMatchEasy0x1K      483739        91226         -81.14%
BenchmarkRegexpMatchEasy1x32      16576         3428          -79.32%
BenchmarkRegexpMatchEasy1x1K      477848        90250         -81.11%
BenchmarkRegexpMatchMediumx32     2093          688           -67.13%
BenchmarkRegexpMatchMediumx1K     517059        95195         -81.59%
BenchmarkRegexpMatchHardx32       73536         13852         -81.16%
BenchmarkRegexpMatchHardx1K       1516938       281635        -81.43%

benchmark                         old MB/s     new MB/s     speedup
BenchmarkRegexpMatchEasy0x32      1.91         9.15         4.79x
BenchmarkRegexpMatchEasy0x1K      2.12         11.22        5.29x
BenchmarkRegexpMatchEasy1x32      1.93         9.33         4.83x
BenchmarkRegexpMatchEasy1x1K      2.14         11.35        5.30x
BenchmarkRegexpMatchMediumx32     0.48         1.45         3.02x
BenchmarkRegexpMatchMediumx1K     1.98         10.76        5.43x
BenchmarkRegexpMatchHardx32       0.44         2.31         5.25x
BenchmarkRegexpMatchHardx1K       0.68         3.64         5.35x
Quickly skip runes that don't match a literal prefix.

With and without:
benchmark                         old ns/op     new ns/op     delta
BenchmarkRegexpMatchEasy0x32      3496          842           -75.92%
BenchmarkRegexpMatchEasy0x1K      91226         13973         -84.68%
BenchmarkRegexpMatchEasy1x32      3428          841           -75.47%
BenchmarkRegexpMatchEasy1x1K      90250         13950         -84.54%
BenchmarkRegexpMatchMediumx32     688           442           -35.76%
BenchmarkRegexpMatchMediumx1K     95195         21300         -77.62%
BenchmarkRegexpMatchHardx32       13852         14134         +2.04%
BenchmarkRegexpMatchHardx1K       281635        272032        -3.41%

benchmark                         old MB/s     new MB/s     speedup
BenchmarkRegexpMatchEasy0x32      9.15         37.99        4.15x
BenchmarkRegexpMatchEasy0x1K      11.22        73.28        6.53x
BenchmarkRegexpMatchEasy1x32      9.33         38.01        4.07x
BenchmarkRegexpMatchEasy1x1K      11.35        73.40        6.47x
BenchmarkRegexpMatchMediumx32     1.45         2.26         1.56x
BenchmarkRegexpMatchMediumx1K     10.76        48.07        4.47x
BenchmarkRegexpMatchHardx32       2.31         2.26         0.98x
BenchmarkRegexpMatchHardx1K       3.64         3.76         1.03x