Performance tweaking rotate.

rotate is a critical algorithm because it is often used by other algorithms,
both std and non-std.  The main thrust of this optimization is a specialized
algorithm when the 'distance' to be shifted is 1 (either left or right).  To my
surprise, this 'optimization' was not effective for types like std::string.
std::string favors rotate algorithms which only use swap.  But for types like
scalars, and especially when the sequence is random access, these new
specializations are a big win.  If it is a vector<size_t> for example, the
rotate is done via a memmove and can be several times faster than the gcd
algorithm.

I'm using is_trivially_move_assignable to distinguish between types like int and
types like string.  This is obviously an ad-hoc approximation, but I haven't
found a case where it doesn't give good results.

I've used a 'static if' (with is_trivially_move_assignable) in three places. 
Testing with both -Os and -O3 showed that clang eliminated all code not be
executed by the 'static if' (including the 'static if' itself).

llvm-svn: 161247

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