sort: deduplicate inner loop of Stable

sort: deduplicate inner loop of Stable

benchmark                   old ns/op      new ns/op      delta
BenchmarkStableInt1K        117212         116287         -0.79%
BenchmarkStableInt64K       9632002        9587872        -0.46%
BenchmarkStable1e4          40044309       39865644       -0.45%
BenchmarkStable1e2          126985         126456         -0.42%
BenchmarkStableString1K     389774         391052         +0.33%
BenchmarkStable1e6          8183202516     8157693442     -0.31%

Change-Id: I14e518ad49ecce3d1fc2b056e1acd5e5a2de8144
Reviewed-on: https://go-review.googlesource.com/1821
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>