[LV] Make the new getter return a const reference. NFC

LoopVectorizationLegality holds a constant reference to LAI, so this
will have to be const as well.

Also added missed function comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272851 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed