[SLP] Treat SelectInsts as reduction values.

Summary:
Certain workloads, in particular sum-of-absdiff loops, can be vectorized using SLP if it can treat select instructions as reduction values.

The test case is a bit awkward. The AArch64 cost model needs some tuning to not be so pessimistic about selects. I've had to tweak the SLP threshold here.

Reviewers: jmolloy, mzolotukhin, spatel, nadav

Subscribers: nadav, mssimpso, aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D13949

llvm-svn: 251424