Changes

[X86] Only look up boolean reduction cost tables if the reduction is not
pairwise.
Summary: We don't pattern match pairwise shuffles in SelectionDAG. So we 
should only return the optimized costs if its not a pairwise shuffle.
I think SLP vectorizer gives priority to non pairwise shuffle if the
cost is the same. And the look up for reduction intrinsics passes false
for the pairwise flag. So this probably has no real effect today.
Reviewers: RKSimon
Reviewed By: RKSimon
Subscribers: hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D69083