A bunch of minor optimizations.

With and without (helps a lot on the "hard" cases):
benchmark                         old ns/op     new ns/op     delta
BenchmarkRegexpMatchEasy0x32      1366          1205          -11.79%
BenchmarkRegexpMatchEasy0x1K      41247         33188         -19.54%
BenchmarkRegexpMatchEasy1x32      1368          1204          -11.99%
BenchmarkRegexpMatchEasy1x1K      40968         34128         -16.70%
BenchmarkRegexpMatchMediumx32     247           240           -2.83%
BenchmarkRegexpMatchMediumx1K     71019         43038         -39.40%
BenchmarkRegexpMatchHardx32       490462        118438        -75.85%
BenchmarkRegexpMatchHardx1K       23500015      5618752       -76.09%

benchmark                         old MB/s     new MB/s     speedup
BenchmarkRegexpMatchEasy0x32      23.42        26.54        1.13x
BenchmarkRegexpMatchEasy0x1K      24.83        30.85        1.24x
BenchmarkRegexpMatchEasy1x32      23.38        26.57        1.14x
BenchmarkRegexpMatchEasy1x1K      24.99        30.00        1.20x
BenchmarkRegexpMatchMediumx32     4.04         4.16         1.03x
BenchmarkRegexpMatchMediumx1K     14.42        23.79        1.65x
BenchmarkRegexpMatchHardx32       0.07         0.27         3.86x
BenchmarkRegexpMatchHardx1K       0.04         0.18         4.50x