Some bounds checking can be eliminated for extra speed

[Marreck]

I have been looking at the performance of this package, specifically at bounds check elimination that was introduced since Go 1.7.

Using go build -gcflags="-d=ssa/check_bce/debug=1" it seems there is a lot of bounds checking going on. Eliminating might in some cases increase performance so I took a quick look if there is some low hanging fruit to be found. There is 1 minor change that can be implemented that impacts performance quit a lot in the 10 Percent hitrate benchmark on my machine:

Before:

goarch: amd64
pkg: github.com/Marreck/intintmap
BenchmarkIntIntMapFill-4                   	      10	 158195310 ns/op
BenchmarkStdMapFill-4                      	       5	 281591720 ns/op
BenchmarkIntIntMapGet10PercentHitRate-4    	   20000	     70048 ns/op
BenchmarkStdMapGet10PercentHitRate-4       	   20000	     65797 ns/op
BenchmarkIntIntMapGet100PercentHitRate-4   	     500	   3021940 ns/op
BenchmarkStdMapGet100PercentHitRate-4      	     100	  10399684 ns/op
PASS

After:

goarch: amd64
pkg: github.com/Marreck/intintmap
BenchmarkIntIntMapFill-4                   	      10	 159295270 ns/op
BenchmarkStdMapFill-4                      	       5	 282393180 ns/op
BenchmarkIntIntMapGet10PercentHitRate-4    	   20000	     58797 ns/op
BenchmarkStdMapGet10PercentHitRate-4       	   20000	     67847 ns/op
BenchmarkIntIntMapGet100PercentHitRate-4   	     500	   2955890 ns/op
BenchmarkStdMapGet100PercentHitRate-4      	     100	  10299620 ns/op
PASS