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/1821Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/sort/sort.go

@@ -359,21 +359,16 @@ func symMerge(data Interface, a, m, b int) {
 
 	mid := a + (b-a)/2
 	n := mid + m
-	start := 0
+	var start, r int
 	if m > mid {
 		start = n - b
-		r, p := mid, n-1
-		for start < r {
-			c := start + (r-start)/2
-			if !data.Less(p-c, c) {
-				start = c + 1
-			} else {
-				r = c
-			}
-		}
+		r = mid
 	} else {
 		start = a
-		r, p := m, n-1
+		r = m
+	}
+	p := n - 1
+
 	for start < r {
 		c := start + (r-start)/2
 		if !data.Less(p-c, c) {
@@ -382,7 +377,7 @@ func symMerge(data Interface, a, m, b int) {
 			r = c
 		}
 	}
-	}
+
 	end := n - start
 	rotate(data, start, m, end)
 	symMerge(data, a, start, mid)