sort: reduce leaf calls in Stable
Move the symMerge recursion stopping condition
from the beginning of symMerge to the callers.
This halves the number of calls to symMerge
while running 'go test sort'.
benchmark old ns/op new ns/op delta
BenchmarkStable1e6
8358117060 7954143849 -4.83%
BenchmarkStable1e4
40116117 38583285 -3.82%
BenchmarkStableInt1K 119150 115182 -3.33%
BenchmarkStableInt64K
9799845 9515475 -2.90%
BenchmarkStableString1K 388901 393516 +1.19%
BenchmarkStable1e2 124917 123618 -1.04%
Change-Id: I7ba2ca277f213b076fe6830e1139edb47ac53800
Reviewed-on: https://go-review.googlesource.com/1820
Reviewed-by: Robert Griesemer <gri@golang.org>