Detecte vector reduction operations just before instruction selection.

(This is the second attemp to commit this patch, after fixing pr26652 & pr26653).

This patch detects vector reductions before instruction selection. Vector
reductions are vectorized reduction operations, and for such operations we have
freedom to reorganize the elements of the result as long as the reduction of them
stay unchanged. This will enable some reduction pattern recognition during
instruction combine such as SAD/dot-product on X86. A flag is added to
SDNodeFlags to mark those vector reduction nodes to be checked during instruction
combine.

To detect those vector reductions, we search def-use chains starting from the
given instruction, and check if all uses fall into two categories:

1. Reduction with another vector.
2. Reduction on all elements.

in which 2 is detected by recognizing the pattern that the loop vectorizer
generates to reduce all elements in the vector outside of the loop, which
includes several ShuffleVector and one ExtractElement instructions.


Differential revision: http://reviews.llvm.org/D15250

llvm-svn: 261804