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         32430         -21.38%
BenchmarkRegexpMatchEasy1x32      1368          1191          -12.94%
BenchmarkRegexpMatchEasy1x1K      40968         32490         -20.69%
BenchmarkRegexpMatchMediumx32     247           240           -2.83%
BenchmarkRegexpMatchMediumx1K     71019         40530         -42.93%
BenchmarkRegexpMatchHardx32       490462        110919        -77.38%
BenchmarkRegexpMatchHardx1K       23500015      5487409       -76.65%

benchmark                         old MB/s     new MB/s     speedup
BenchmarkRegexpMatchEasy0x32      23.42        26.55        1.13x
BenchmarkRegexpMatchEasy0x1K      24.83        31.57        1.27x
BenchmarkRegexpMatchEasy1x32      23.38        26.86        1.15x
BenchmarkRegexpMatchEasy1x1K      24.99        31.52        1.26x
BenchmarkRegexpMatchMediumx32     4.04         4.16         1.03x
BenchmarkRegexpMatchMediumx1K     14.42        25.26        1.75x
BenchmarkRegexpMatchHardx32       0.07         0.29         4.14x
BenchmarkRegexpMatchHardx1K       0.04         0.19         4.75x