sort: simplify rotate and reduce calls to it

Move the checks for empty rotate changes
from the beginning of rotate to the callers.

Remove additional variable p used instead of existing m with same value.

Remove special casing of equal ranges (i==j) to exit early as no
work is saved vs checking (i!=j) and  making a single
swapRange call if this is false.

benchmark                   old ns/op      new ns/op      delta
BenchmarkStableString1K     417195         425218         +1.92%
BenchmarkStableInt1K        126661         124498         -1.71%
BenchmarkStableInt64K       10365014       10417438       +0.51%
BenchmarkStable1e2          132151         130648         -1.14%
BenchmarkStable1e4          42027428       40812649       -2.89%
BenchmarkStable1e6          8524772364     8430192391     -1.11%

Change-Id: Ia7642e9d31408496970c700f5843d53cc3ebe817
Reviewed-on: https://go-review.googlesource.com/2100
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>