Reworks nilcheck to be performed by a depth first traversal of the
dominator tree, keeping an updated map of the values that have been
nil-checked during the traversal.

benchmark                           old ns/op     new ns/op     delta
BenchmarkNilCheckDeep1-8            1242          1825          +46.94%
BenchmarkNilCheckDeep10-8           2397          3942          +64.46%
BenchmarkNilCheckDeep100-8          29105         24873         -14.54%
BenchmarkNilCheckDeep1000-8         2742563       265760        -90.31%
BenchmarkNilCheckDeep10000-8        335690119     3157995       -99.06%

benchmark                           old MB/s     new MB/s     speedup
BenchmarkNilCheckDeep1-8            0.81         0.55         0.68x
BenchmarkNilCheckDeep10-8           4.17         2.54         0.61x
BenchmarkNilCheckDeep100-8          3.44         4.02         1.17x
BenchmarkNilCheckDeep1000-8         0.36         3.76         10.44x
BenchmarkNilCheckDeep10000-8        0.03         3.17         105.67x

benchmark                        old allocs     new allocs     delta
BenchmarkNilCheckDeep1-8         9              14             +55.56%
BenchmarkNilCheckDeep10-8        9              23             +155.56%
BenchmarkNilCheckDeep100-8       9              113            +1155.56%
BenchmarkNilCheckDeep1000-8      9              1015
+11177.78%
BenchmarkNilCheckDeep10000-8     9              10024
+111277.78%

benchmark                        old bytes     new bytes     delta
BenchmarkNilCheckDeep1-8         432           608           +40.74%
BenchmarkNilCheckDeep10-8        1008          1496          +48.41%
BenchmarkNilCheckDeep100-8       8064          11656         +44.54%
BenchmarkNilCheckDeep1000-8      73728         145240        +96.99%
BenchmarkNilCheckDeep10000-8     737280        2144411       +190.85%

Change-Id: I0f86010e9823aec04aac744fdb589b65ec8acefc
Reviewed-on: https://go-review.googlesource.com/12332Reviewed-by: David Chase <drchase@google.com>