[LSV] Use a set rather than an ArraySlice at the end of getVectorizablePrefix. NFC

Summary: This avoids a small O(n^2) loop.

Reviewers: asbirlea

Subscribers: mzolotukhin, llvm-commits, arsenm

Differential Revision: https://reviews.llvm.org/D23473

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278581 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed