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/1820Reviewed-by: Robert Griesemer <gri@golang.org>